Master cylinder or brake booster or ?

[rjfurlan]

67 250sl replaced rotors, calipers, hoses and pads. Bled brakes. Started car pushed brakes and they went to the floor. I noticed the plunger at the end of the brake booster (picture) goes in but does not come out. I can pry it back out and the brakes feel ok when the car is off, when the car is running the brake peddle goes right to the floor and the plunger stays in. Does that indicate a bad master cylinder or brake booster or something else?

Thanks
Rich

[Raymond]

Sorry.  Your sentence is confusing.  "...brakes feel ok when the car is off on right to the floor and the plunger stays in."   

[rjfurlan]

I press on the brakes and I can push them all the way to the floor and the plunger in the back of the brake booster does not come back out.

[Benz Dr.]

Check that the lever sitting inside of the aluminum casting isn't siezed to the pin it pivots on. Not common but I've seen it before.

[rjfurlan]

Thanks, it is not siezed. When the car is off the brakes seem ok. After I start the car the brakes peddle goes to the floor easily. Is that a sign of a bad master cylinder?

[Benz Dr.]

Kind of sounds like MC problems. Make sure you have all of the air bled out of the system.

[rjfurlan]

I rebled the brakes again. I also bench bled the MC on car. Same situation. The brakes seem to work when car is off. When I turn the motor on the brake peddle goes down easily but does not come back up. It will go all the way to the floor. When I turn the car off and pull the rod back out the brakes seem fine. I do not see any leaking anywhere. Did I brake the MC by pushing down too far?

I have a new MC so will try that. I am not sure it is the MC but do not have any other ideas.

[johndillman]

I hate to say it, but still sounds like air to me.  I would rebleed, then rebleed, etc.  If the master isn't bench bled at the beginning, and the OP wasn't entirely clear on that, all that air has to go somewhere, and it has to make it ALL the way through the lines .. . My '65 Vette drove me crazy with bleeding once.  It takes a lot of time . .
 
John

[Benz Dr.]

Usually, a bad MC will still hold some pressure but will bleed down quickly, or not, but the fact that it won't hold is the main concern. No pedal at all does sound like air in the system.

 We had to replace a brake booster this week because the brakes felt spongy. Even though the MC was new and we bled the brakes carefully three times; it didn't feel right. I put a used booster on the car and the pedal was very firm right from the start - too firm in this case.
I usually set the eccentric bolt on the clevice to the full position ( notch facing the firewall ) but on this car it made the brakes lock up. A quater turn so the notch was pointing straight up was all it took. Car has excellent brakes now. So, don't discount the booster.

[rjfurlan]

Ok, I installed a new master cylinder, bench bled, and bled the brake lines. The brakes work but part of the problem remains. When I start the car and push on the brakes, they work, but the peddle does not return and the brakes remain on. If it pull on the push rod it comes out and the brakes are off. Is the booster not releasing? Is something in the booster broken?

[rjfurlan]

Ok, maybe it is the bottom pivot on the booster. I have wd40'd it and tried silicon grease. Did not seem much different. How do i make sure the bottom pivot is free? I took the cotter pin out but the pin will not move or come out. Now what should I do?

[tel76]

Remove all three parts, not a difficult job.

[rjfurlan]

Remove all three parts, not a difficult job.

? Which three parts? After I take out the cotter-pin, how do I get the lower pin out. Is there a nut on the end. On mine it looks like a pin with a hole in the end. The epc shows a nut on the end but i do not see a nut on mine.  Should I try to pound it out? Or is there a stripped nut on the end.

[ejboyd5]

Plug the vacuum source to the booster, start the engine and tell us how the brakes work.

[tel76]

Remove master cylinder, brake booster and item# 172, you will then be able to remove the lower pin very easily.

[rjfurlan]

Remove master cylinder, brake booster and item# 172, you will then be able to remove the lower pin very easily.

Thanks. I took everything off and finally got the pin out. Cleaned everything, reinstalled and I finally have working brakes!

[Benz Dr.]

So the lever was siezed on the pin?

[rjfurlan]

So the lever was siezed on the pin?

The lever was moveable, but very difficult, so it was not totally siezed. Once cleaned and reinstalled, it moved freely. The master cylinder was also bad, as well as the brake hoses, a few metal lines, the calipers, and the rotors. Also replaced fuel pump. The car sat for 8 years and today was the first time it has been driven. Drove about 5 miles. I have been working on it about 3 months. Thanks for the help.

[Benz Dr.]

That lever has to be completely free to move or you will have brake trouble. Maybe not the first place anyone would look but when all four wheels are locking up, it would be a good place to start. Also check the eccentric on the push pin. There should be a small amount of free play with the high spot adjusted as far back as you can get it. Look for the small notch which can be found on almost all units and turn it so that it points towards the firewall. If you don't feel any free play after doing this adjustment turn it ahead a little bit until it frees up. The idea is to remove eccess brake pedal travel so that you never have to pump your brakes to get your car to stop. Or, worded another way, the brake pedal remains firm at all times.

[tel76]

If it had not been brakes I would have let the above comments ride, but I must take issue with the correct reason you have to have this free play.
The free play is required to ensure that the piston in the master cylinder fully returns to its stop ie the circlip that retains the piston in the master cylinder.
If you do not allow the piston to return fully the small hole in the master cylinder body that allows the brake fluid to return to the reservoir (after brake release) will be covered or partially covered, if the hole is partially covered then the release of pressure will be slow resulting in slow brake release, if the hole is fully obstructed then the brakes will lock on.

[rjfurlan]

That lever has to be completely free to move or you will have brake trouble. Maybe not the first place anyone would look but when all four wheels are locking up, it would be a good place to start. Also check the eccentric on the push pin. There should be a small amount of free play with the high spot adjusted as far back as you can get it. Look for the small notch which can be found on almost all units and turn it so that it points towards the firewall. If you don't feel any free play after doing this adjustment turn it ahead a little bit until it frees up. The idea is to remove eccess brake pedal travel so that you never have to pump your brakes to get your car to stop. Or, worded another way, the brake pedal remains firm at all times.

Thanks Benz Dr., good tip on adjusting the eccentric screw.