Brake Master Cylinder Symptoms

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Reader Question
My mechanic said that my brake master cylinder needs to be replaced soon. How can he tell? I brought my car in for other repairs, and I don’t know if
he is just trying to sell an add-on job.



Hey there Jake

So you are starting to feel uneasy about your mechanic’s recommendation? How do you know if he is doing his job correctly and looking after your best interests and your safety?

When a brake master cylinder begins to go bad or fails, you will notice a soft or squishy feeling when you press down on the brake pedal. When constant pressure is maintained on the brake pedal (like when you keep your foot on it at a stop light), the brake pedal will begin to sink to the floor as the brake fluid leaks internally in the master cylinder.

This is called “extended travel” when the brake pedal goes farther down than normal or than it was intended to. When this occurs you will have to “pump” the brake pedal to regain normal pressure and to keep the car from moving forward because the brakes are slowly releasing.

The red brake warning dash light should come on to indicate low brake fluid, or
excessive movement “travel” in the brake pedal.

You will not normally see the brake fluid leaking out externally of the brake master cylinder or from the wheel areas when this pedal softness occurs, so fluid level alone isn’t an indication of a good or bad brake master cylinder.

The fluid will usually not be low or in need of topping off remember the leak is
internal and the brake fluid is leaking past internal O-rings, so an obvious external sign that the master cylinder is “bad” is usually not existent.

Ok, that is great Austin, but that is not happening in my case. What else could have tipped off this mechanic to a possible faulty brake master cylinder? An external brake fluid leak could be possible, and would be visible to the mechanic as he was performing his usual under hood inspection.

The master cylinder usually has a plastic reservoir that holds the brake fluid, Click for example, and this reservoir is mounted to the metal part of the
master cylinder by rubber grommets.

The grommets allow for some movement caused by brake pedal and fluid pressures. These grommets can leak fluid, and a visible brake fluid leak can be seen with the naked eye (brake fluid is a clear liquid).

The metal brake lines that are attached to the master cylinder can leak fluid at the threaded connections, Click for example. Brake fluid can also leak from the back of the master cylinder due to these internal O-rings we talked about earlier.

When this happens, the fluid will drip down the power brake booster (the booster is what the master cylinder is bolted to), Click for example. Brake fluid will dissolve paint rather quickly, so this kind of leak will usually leave a tell-tale sign of blistering, bubbled paint under the brake master cylinder.

So if you experience any of the symptoms explained above, give your mechanic a “brake.” He was doing his job thoroughly and was trying to help. In any case, check the brake fluid level first and determine if fluid is needed.

Check the cap and make sure it is fitted snuggly on the cylinder and is not leaking or showing any obvious signs of past leakage. Visually inspect the cylinder externally for fluid leaks, and wipe any accumulated dirt and debris
from the area and the cylinder for ease of future inspections.

The cylinder should only need a very small amount of brake fluid as part of
regular maintenance, so if more than an ounce or two is needed to top off the reservoir, you should have your brake system inspected by your mechanic. Do not put off any needed repairs to your braking system.

I have put together some easy-to-follow maintenance schedules with more recommendations and explanations that are free to view and print out from our  website.  Vehicle Maintenance Schedules.

Austin Davis

22 Responses to Brake Master Cylinder Symptoms

  1. Hey Paul, I think….if you want to take one more guess, I think I would replace the master cylinder…but its a guess. Make darn sure the tires on the rear are in good shape. My wife’s SUV had 32K on them and the front brakes would lock up (abs control took over) on slightly wet pavement. Althought there was still some tread left, the tires were a little thin. Replaced 2 front tires and issue went away.

    Your brake system is pretty simple, so dont over look the obvious…tire worn thin and loosing traction on stops, brake fluid leak on the brake pads, rear axle bent due to hitting a curb etc. causing misalignment, brake master cylinder sticking…although I would really expect both rear wheels to be affected. I would raise the rear end up, spin each wheel by hand, apply the brakes with engine running, then respin the rear wheels to make sure brake was fully released.

  2. Paul Lozier says:

    I have a 2002 mercury grand marquis with traction control and ABS it has a grabbing issue, intermittent. I have traced it to back wheels, 4 wheel disc system. I have replaced all 4 rotors, pads all around, rear calipers, and rear parking brakes. I have cleaned all ABS sensors and i have also shut Abs system in several ways, removing fuse for pump, unhooking pump harness, and shutting traction switch on dash. traction lights test on start-up as do ABS lights. As we drive the car grabs like the brakes are locking up then just release, usually 1 to 2 seconds. Sometimes I smell the brake heating after a long trip. Recently after changing rear calipers as you come to a stop it seems as though the mechanical backup in master cylinder is kicking in. The only components left in system that affects rear wheels are master cylinder and Traction / ABS unit. I can not figure where issue might be as it only seems to happen on 1 wheel, as in the TCS / ABS unit hanging. Can you suggest a plan of action?

  3. I would try “gravity” bleeding the brake fluid out of the system for an hour or two and see if that helps. You should be able to find a few Youtube videos on how to do it properly.

  4. Lilliannet Rosario says:

    We have soft breaks and we don’t know what else to do. We replaced the master cylinder, our lines are not leaky, we have bled the breaks, two man job, 4 times already! What else can we do?

  5. Usually a real hard brake pedal is due to lack of vacuum assist from the brake booster, either an internal problem inside the booster itself OR lack of vacuum from the engine to the booster. There is a plastic check valve on the end of the rubber vacuum hose that plugs into the brake booster. With the engine running you can remove that hose/valve and the engine will race up real high, or sometimes die due to this vacuum leak you just created. If nothing changes, then the check valve could be bad or you have a vacuum issue on/from the engine that needs to be checked out.

    A soft brake pedal or a very low brake pedal can be due to a fluid leak at the wheels or brake lines/hoses or could be an internal problem inside the brake master cylinder, which needs to be replaced. Make sure there are no leaks at all 4 wheels first, check vacuum to booster, then things might be pointing to a master cylinder issue if everything else checks out. You can sometimes hear a hissing noise under the dash at the brake pedal with the engine running if there is an internal booster problem, so listen for that as well. I would also make sure there is not a differential grease leak that has coated the rear brake shoes with grease, causing them to slip and drag on the brake drum. Pull rear wheels off and check for grease on the brake shoes.

  6. Dee says:

    I drive a 2001 dodge ram 1500 5.2liter engine, and I’m having issues with my brakes. One day I noticed my brake pedal got hard, and I took it to the shop. I was told my brake booster needed to be replaced. So I got it done, got new brake pads, also got the booster adjusted. However, now my brakes go from being hard, to soft, to hard again. No leaks that I could see from the mc, although I replaced the gasket on the cap that was worn and flimsy. Also had the brakes bled. What else could it be. The truck brakes, but the pressure I need to apply changes from stop to stop.

  7. wow, you have replaced just about everything. Does Wilwood offer a help hotline? Did they supply the booster and master cylinder to fit this application? The only thing I can think of off hand, or what comes to mind first is a parts issue not being compatible with those brake calipers???? I have never adapted anything like this so I dont have any first hand knowledge as to what could be the problem. I would get them on the phone.

  8. danny says:

    working on 89 mustang with wilwood brakes all 4 disc when i try out car the brakes stay applied I can jack up any wheel and they come lose but if i drive 100 feet it does it again new master cly new booster new lines new porp vavle really could use help

  9. Austin Davis says:

    Hi there,

    Yes, they did have to remove the brake caliper from the brake rotor to get access to the wheel bearing but they did not need to take the brake fluid line off…so it was just removing the two mounting bolts from the caliper to move it. I would not really expect there to be any “related damage” that they could or would have caused during that time to cause this issue now.

    If the brake fluid is full, then there are no leaks. “Pumping” the brake pedal a few times should regain pressure, and as you hold you foot on the brake pedal it will slowly sink to the floor again…correct? If so, that is a pretty good indication of a bad brake master cylinder that is leaking fluid internally and needs to be replaced.

  10. ann says:

    have had problems with brake pressure after a wheel bearing was changed. Brakes are less than 6months old, no problems before, but within a week after bearing started loosing pressure, have to press pedal all the way to floor to stop. Fluid full, can’t see any leaks, no lights come on. mechanic said they touched nothing with the brake hydraulics. but still could be a leak they cant find or air & want to charge a diagnostic check to find the problem. Everything I researched mentions moving caliper, ABS/sensor, so how did they change bearing without touching no brake parts, could they have missed or damaged something to cause this problem. The seal on the master cylinder is bad? HELP

  11. Austin Davis says:

    Thanks for your kind words.

    I would remove all 4 wheels and re-inspect all 4 wheel brake parts. You should NOT be low on brake fluid, especially after a recent brake job so you probably have a brake fluid leak somewhere. The symptoms you talk about are common with a bad master cylinder, which is called “by passing” BUT since the brake fluid level is low….I would suspect a leak somewhere like one of the new wheel cylinders causing the problem. Go back to the original brake mechanic and have them re-inspect their work.

    The mechanic who bled out the system and let you go, I don’t think that person is a real mechanic and would not go back to him if possible. If there are NO external leaks, then yes a new master cylinder is probably in order.

    Keep us posted

  12. Melissa says:

    First of all, I want to tell you that your blog is very helpful. I got told that my master cylinder should be replaced today. A month a go I got my brake disc resurfaced, new brake pads for front and rare, brake shoe, wheel cylinder, inner tie rod (both side) all changed (because the Brake light was on every time I pressed the brake and I had to pump the brake to get my car stop). I could stop my car very well for a couple weeks. Then I had to press further and further to get my car stop, and finally I felt that I must press the brake the deepest to get my car stop. So I went to get it checked at repair shop, and they told me that it needed to be bleeding. So they bled the air out. It went back to great again only one day then it started to need more pressing to stop. I checked the brake fluid reservoir and it was in Min level. So I thought it might be low brake fluid so the brake needed more force to press and my car could not stop. On the way to the repair shop, the brake light went on every time I pressed the brake (again). The mechanic said the master cylinder needed to be replaced, but I am still wondering if I need to just get it replaced or should fill brake fluid first and see if light still goes on. Please advise.

  13. Austin Davis says:

    Hey Paul, Yes, you are correct on the hard pedal and possible booster issue…that is USUALLY the case. It just seems you have pretty much replaced everything, and you are obviously experienced, so I am just trying to rule out anything you might be over looking.

    Like I said, we had a similar issue and we replaced the booster and it solved the problem. Now, was it due to air trapped in the master and the complete booster/master helped solve our air problem…probably. Do you have a bad master….maybe, but I kinda doubt it.

    Usually you want to bleed the master on the bench, but since you are dealing with a long brake line system you might have to bleed it on the vehicle.

    I talked with my top tech this morning, and he chimed in with this below.

    I wonder if he has rear anti-lock brakes…if so,the dump valve assembly for the rear brakes may be by-passing…this happened on a-lot of trucks…other experiments would include pinching front or rear lines and see if a firm pedal returns…also plugging the master cylinder lines..looking for a firm pedal..just trying to isolate different parts of the system

  14. Paul Dickie says:

    I always assumed that bad booster would be a hard pedal not a soft pedal but i did notice the other day that when i had another tech pumping the brakes i was seeing small amounts of bubbles in master making me think air in master. i never saw bubbles before but quite possible i overlooked as master was just bench bled and installed.i did bench bleed this master when installed but wondering if you have ever had any luck bleeding a master on vehicle as this application does not have a bleeder screw.thinking that my whole problem is air trapped in master but is that possible after vacuum bleeding upwards of 5 qts through system?

  15. Austin Davis says:

    We had a similar issue a few years ago with a slightly older motor home like this and we had to replace the booster. We really don’t work on stuff like this, so it could have just been a lucky guess. We ordered it loaded with the master cylinder, so you might have a bad master or maybe you have a booster issue as well.

    If I remember correctly, we bought the booster master combo rebuilt from A1 at our local part supplier. Worth a shot I guess, you have replaced and done everything else. So you either bought a defective master….doubtful but possible or for some reason the booster is causing the problem.

  16. Paul Dickie says:

    I have a problem with my brakes im hoping you can help with. I am an ase and evt certified tech but never to proud to ask for help. I recently purchased a 88 allegro motorhome on a gmc p3500 chassis. the rear brake line had rotted off from sitting and the system had run dry with the master cylinder cover removed. When i bought it the first thing i did is replace bad line and master cylinder as master was left exposed to elements. I also replaced all calipers and pads/rotors and replaced proportioning valve as that was left dry for extanded time. tried bleeding brakes but got no fluid fron front calipers so replaced front caliper rubber hoses and now got fluid from all 4 wheels. bleed all 4 wheels till i had clear fluid, nor air present, with vacuum bleeded but pedal spongy. brakes will stop vehicle when put into drive at an idle but barely, dont dare try to move it under power. i then replaced rear caliper rubber hoses as they were cheap and felt possible internal failure as with the fronts. when rear rubber lines were installed i opted for the traditional pump and hold way of bleeding to bleed incase using vacuum bleeder may not work due to check valves in proportioning valve. would also like to note i have and use proportioning valve set tool to lock proportioning valve check valves when bleeding. i now have all new lines, master, proportioning valve, calipers; essentially completely brand new brake system but spongy pedal to the point where its hardly safe to move in the driveway. Anything i may be overlooking? i am thinking that i may have faulty master cylinder or possibly air trapped in master cylinder. I did bench bleed master before installation but your thoughts would be appreciated. Thank you in advance, Paul

  17. Austin Davis says:

    Thanks for your comments Jose.

    Hummm, USUALLY…usually when you have to pump the brake pedal it means either there is air in the brake lines or the master cylinder is “by-passing” which means there is internal failure inside the master cylinder.

    Did you use a new cylinder or a remanufactured one. I have not had much luck with the cheaper rebuilt units and would suggest trying a new master cylinder. Bendix, Wagner and Raybestos are name brands you can buy at your local auto parts store.

    If you used a new cylinder, then I would suspect there is air still trapped in the lines that needs to be bled out. I have seen 1 instance where a booster was the problem, but usually with a bad booster you get a very hard pedal that is very hard to press down. You can also hear a hissing noise under the dash and or under the hood with the engine running.

    I think I would go back to the shop that did the inspection and said you had a defective cylinder and let them try and bleed the system for you…..or tell you what is wrong, again. :(

    I would also double check the other work your friend did…brake pad installation, brake rotor install etc. etc. It’s a pretty simple system, but sometimes it’s the simple OVERLOOKED items that cause all the problems. You can’t just assume the other items are installed correctly.

    Keep me posted

  18. Jose Hernandez says:

    I thoroughly enjoy your honesty in helping us non-mechanically inclined folks. So I ran into this same issue a couple of months ago when I took the car in for some repairs and was told that my van needed a new master cylinder. instead of paying the dealership I got a friend to do it for me for less than half the price. He even threw in changing the front brakes and the rear rotors. However, after checking everything, I still have to double pump on the brakes to actually be right.
    I took it to an automotive place to get it inspected to find the issue. After bleeding out the brakes and seeing that it wasn’t leaking anywhere, they stated it was a bad cylinder.
    Again trying to save money, I changed the master cylinder myself. I used a vacuum to suck out the air. After that didn’t work, I did the old fashion way and let gravity do its work. It still has the same issue where I have to double pump the brakes for the pedal not to go to the floor. I thought it might be the booster but that was checked and its fine. Any advise or assistance would be greatly appreciated.

  19. Austin Davis says:

    Tina, why did you need to replace all brake lines? Rust? That is not something that normally gets changed, unless they have rusted out and are leaking fluid. I would first bleed them again and again, sometimes it takes a few times to get all the air out.

    Best way to do it is with a vacuum pump that sucks out the fluid/air, but if you dont have one you should try to gravity bleed the system. Open one line at a time, start with R/R and let gravity suck out the fluid…about 15 minutes, then repeat on other wheels.

    Mashing down on the brake pedal or pumping the brake pedal is the old school way to bleed, and can cause damage to the master cylinder as you cram rust and junk deep inside the master cylinder as you push the pedal all the way to the floor.

    Make sure nothing else is leaking, hose, caliper, wheel cylinder if you have rear drum brakes etc.

    Replacing the master cylinder would be my last recommendation, but it is possible there is a problem with it. Hopefully there is still air in the system you have not gotten out yet.

  20. Tina says:

    Okay had to replace all the break lines in my 2001 chevy silverdo and now cannot get pressure. What is going on? Do I need to get a new master cylinder?

  21. Austin Davis says:

    Thank you Tyler! Glad to be of assistance.

  22. Tyler says:

    I really love this blog. You write about very interesting things. Thanks for all your tips and information.

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