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Trying to figure out what's going on here and wanted some input so I don't start just throwing parts at this problem. Replaced front drum brakes with disk brakes over the winter. Added a proportioning valve but kept my original booster and master cylinder as they were fine before the conversion. Bled it and got a good stiff pedal. In the last week after putting over 1000 miles on the car and having no issues with the brakes a problem has developed - I'll be sitting at a stop light and sometimes the brake pedal will starting sinking to the floor and the car will creep until I add additional pressure. Note that I don't have to pump the brakes to get good stopping power and I have a stiff pedal until I'm stopped and run into this issue. Checked the master cylinder fluid levels and noticed that the rear reservoir was slightly lower than I remember but nothing large (maybe 1/4 inch). Checked all of the line connections and no leaks. Checked the rear cylinders and no leak there either. Bought a new check valve thinking it may be that but it made no difference.
Based on the symptoms, does this sound like a master cylinder problem or a booster problem? Or could it possibly be a failure of the new proportioning value?
Internal leakage of the master cylinder. When was the last time that the master cylinder was serviced? How often do you change brake fluid? If your fluid is brown, that is a good indication that it is past due for a complete fluid change of the system. Brakes in my opinion are the most overlooked system of the car, and second are worn steering parts. I don't know why people don't pay more attention to these two items, because they are more important than any other part of the car, when it comes to safety. A bad engine, transmission, or differential will leave you at the side of the road, but bad brakes and steering have the potential to kill you.
Your probably leaking fluid into the vacuum booster. Unbolt it from the booster and check.
Was thinking that too. Assuming I am that probably means both the master cylinder and the booster need replacing, correct? MAW do both at this point....
Internal leakage of the master cylinder. When was the last time that the master cylinder was serviced? How often do you change brake fluid? If your fluid is brown, that is a good indication that it is past due for a complete fluid change of the system. Brakes in my opinion are the most overlooked system of the car, and second are worn steering parts. I don't know why people don't pay more attention to these two items, because they are more important than any other part of the car, when it comes to safety. A bad engine, transmission, or differential will leave you at the side of the road, but bad brakes and steering have the potential to kill you.
Thanks Tedd and Junkman. I'm pretty sure the master cylinder is original to the car so I MAW replace it. I vacuum bled the brakes when I did the disk upgrade so the fluid is pretty fresh. Replaced all steering components as well, so item 2 on the list was addressed over the winter too.
The booster may still be ok depending how long its been leaking. Your other issue although unrelated is your using a drum brake master on a disc brake system.
The booster may still be ok depending how long its been leaking. Your other issue although unrelated is your using a drum brake master on a disc brake system.
Thanks. Regarding the master cylinder, there are 2 reservoirs on it and the rear one is smaller that the front. I assumed that the factory just used the disk/drum one for the drum/drum cars. Is there any other difference?
Or fluid is leaking back around the seals in the master and flowing back to the reservoir....Tedd
^^^THIS.
First, drum/drum master cylinders have residual pressure valves in the output ports. Disk brakes do NOT use the residual pressure valve. Change the M/C for a correct one.
Second, if brake fluid were leaking into the booster, that means that the fluid level would be going down and that both the M/C piston seals and the seal between the booster and M/C are bad. More common is a leak out the back of the M/C that shows up as a trail of brake fluid on the outside of the booster under the M/C.
Third, an original M/C likely is worn, causing the internal leak that Tedd talks about.
OK - this is getting more interesting. Oldcutlass and Tedd nailed it as when took off the MC there was fluid against the booster:
I could see a fluid leak at the back of the MC. Bench bled a new (not remaned) MC, connected it, bled the brakes all around, got a firm pedal, and took it out for a test drive. The brake warning light came on. The pedal was still firm. I parked it and noticed that it stayed on until I put pressure on the brake pedal. Then it goes off. Release pressure, and it comes back on and stays on. Check the E-brake pedal and it's all the way up, so that's not it from what I can tell. Brakes seem to work fine. The brake warning light sensor is mounted on the proportioning valve.
Just unscrewed the brake light switch (plunger) on the proportioning valve to see what's going on and there is brake fluid on the hole where it goes. Is my proportioning valve toast?
Last edited by dalilama; July 9th, 2017 at 10:01 AM.
Reason: Grammer
The switch works by a sliding piston. When you bleed the brakes, you will slide the piston to one side or the other. I would open the bleeder at the rear brakes, and have someone else gently give the pedal a push, while you operate the bleeder. This should center the valve, if you bled the front brakes last. Sometimes you have to play with it a little to get it centered. The switch works on a grounding mechanism, and is in contact with the brake fluid.
All this is assuming that they haven't changed the method of turning on the light in the newer switches.
The switch works by a sliding piston. When you bleed the brakes, you will slide the piston to one side or the other. I would open the bleeder at the rear brakes, and have someone else gently give the pedal a push, while you operate the bleeder. This should center the valve, if you bled the front brakes last. Sometimes you have to play with it a little to get it centered. The switch works on a grounding mechanism, and is in contact with the brake fluid.
All this is assuming that they haven't changed the method of turning on the light in the newer switches.
Manually centered the valve with a pick and the light goes out. However as soon as I step on the pedal the light comes on and stays on. There must still be an imbalance in the pressure so I'll re bleed and see if that helps.
July 8th, 2017, 06:17 PM
