Here is the "gory story"! 71 GS rear drum/front disc. It had a weeping master cylinder so I replaced it, and it bench bled well. I had a poor time bleeding the rear brakes as the fluid only trickled out. Under the advisement of a few forum members I was told to depress the residual pressure valve (front of prop valve) which did seem to help but the result was still spongy brakes even though the fronts bled out fine with good pressure as they should. I then replaced the proportioning valve with a OPGI replacement thinking that it may be something goofy going on? It made no difference with poor rear pressure again, so I then depressed the residual pressure valve again with little improvement. I bought a "mighty vac" and bled the system with good results (moved an amount of fluid that must have completely bled the system)? Brakes still go almost to the floor and do not seem to have the bite that they should. The one thing that I don't think is normal is air "puffing" sound that is coming out from under the dash when the brakes are depressed. I don't know if the booster diaphragm is bad or if this is normal or what? The booster has a strong vacuum source and is using the manifold port located behind the carb. Anyone have any insight on this one, I'm at a loss???????
Sounds like you may have a new/ defective master. Was it a new master or a rebuilt? If the only issue previously with the brakes was that there was an external leak, and now you have a pedal that goes to the floor, it seems to point to a bad master. Its not uncommon with rebuilt parts
The first master I got did not bench bleed, the one on the car now bench bled well and seems to move fluid as it should? The puff of air from under the dash does not seem normal to me in this whole situation. I would normally agree with you about the master being the only change but the car is new to me and I didn't really drive it more than off the trailer and 100 ft before it got the new master. The pedal feel prior to this ordeal may have been only slightly better. Is it normal to hear air from the booster under the dash when the pedal is depressed?
Puff of air is the vaccum booster venting and is normal. Some are louder than others, there is usually a wad of fiberglass or jute type stuff that acts as a filter/baffle and if it falls out of deteriorates you will get more noise. Our brake system is pretty damn simple and I would agree that it sounds like a bad master cylinder. If this is a 71 with the integrated valve on the frame then the button does pretty much nothing so pressing it won't help. What I have seen more than a few times through the years is an air bubble that gets stuck in the combo valve and the most reliable solution I have found is to open a front bleeder and JAM the pedal to the floor, close the valve and repeat a few times. Do the same with one of the rear bleeders then bleed as normal. This will usually get the air bubble out into the lines where it will bleed out properly. Please remember if you are bleeding the brakes manually you should generally be opening the bleeder, pushing the pedal down gently, close the bleeder, release the pedal then repeat. Remember to keep an eye on the fluid level in the MC and NEVER DO THAT PUMP THE PEDAL THING!!!
I know it's not a new Mercedes here. WE have been pumping the brakes, then hold down, open bleeder screw, close, pump repeat. We used a "mighty vac" to vacuum a great deal of fluid through all four corners so you would think that any air would be gone, but here we are. The proportioning valve is new, most lines are new, wheel cylinders and calipers are new. The booster one way valve seems good, pressure in the booster, good vacuum source, hose lines etc... I definatly got a bad master the first time and got the replacement from the same place so maybe??????????
I doubt your getting several bad master cylinders, i suspect you still have air somewhere. IMO i would pull all 4 wheels, put hoses on each bleeder, run the hoses into clear containers and open all the bleeders. Let the car sit for a couple hours but be sure to keep the master full of fluid. This one of the best ways besides pressure bleeding to get air out. When finished have someone go around and pressure bleed each wheel at a time starting from the furthest to the closest, also check that none of the brake hoses are swelling under pressure too.
http://www.v8buick.com/showthread.php?76640-bleed-bleed-bleed-no-pedal!!-!&highlight=bleed Lots of good info in the old post link above. Whatever you do when you bleed NEVER "pump up" the pedal! When you pump the pedal up all you do is hammer the lines and disperse the nice big air bubble that would have bled into a ****load of little dispersed bubbles that will cling to the inside of your brake lines/calipers/cylinders/etc that won't move. Also when you did the bench bleed did you tap on the MC housing to knock any air bubble loose? How about with the MC tilted nose up a few strokes and nose down a few strokes?
Thanks for the ideas. I broke down out of frustration and purchased a new master cylinder, not rebuilt. I know the chances are slim of getting two bad masters in a row, but for the price they were the cheap offshore variety, so here goes. I'll bench bleed as suggested and not pump the brakes although I'll likely use the "mighty Vac".
I suggested that he try cracking the steel line before the rear flex hose in his prior thread. Im not sure of it was done or not.
No it wasn't done, but the flex line was in good shape. Anyways I got them fixed tonight, not sure exactly what the change was but I changed the master out for the new one that was bench bled, then used the "mighty vac" to bleed the lines without pumping the brakes. Now I have a good pedal, thank god! Now I can use the car again.
True story- I got 4 bad rebuilt masters in a row. I thought I was losing my mind. The 5th master was a new Bendix. Its still in there
^Is this a typo?^ Try this method: 1) Open bleeder valve 2) slowly press brake pedal to floor 3) close bleeder valve 4) release brake pedal 5) repeat until no air bubbles are seen in fluid
