Brake Question

[Chuckler3]

I've been through the forums, service manuals, and the brains of my friends in search of solutions to my issues, but I have found none.

When I purchased my car, my pedal was dead until the last half inch of its range of motion. That half inch felt like a normal brake pedal and would stop the car.

I replaced the booster, and have since flushed and bled the system almost a dozen times with the same results.

I now have a pedal that goes all the way to the floor.. pumping produces some resistance, but as the pumping slows the pedal inevitably goes to the floor. My line of thinking is that it's the master cylinder. The previous owner had said that he had replaced it, but the caps on the resevoir are cracked and leaked during the pressure bleeding process. I have stopped them from leaking with some expertly placed hose clamps, but the pedal still goes to the floor. I'm prepared to order a new master cylinder, but I wanted to check here first with the collective mind to ensure proper course of action.

[Frank Reiner]

Truck? Motorcycle? Go-Kart? Airoplane? Snowmobile? Automobile?

[Chuckler3]

Superman...

1982 300SD.. no ABS.

My bad.

[Frank Reiner]

I think that my first investigation would be to determine if the mechanical parts of the master cyl./booster/pedal package are correct. With the engine off, i.e., no boost from vacuum, is resistance from the M/C felt with less than 1/4 inch of pedal travel (free play)? If yes, do you then still experience a long pedal? If, however, there is a free pedal travel of over an 1 1/2 inch, there is a likely mismatch of push rod/booster/master cyl.

If the free travel of the pedal is normal, try bleeding just the M/C by alternately loosening the lines at the M/C and stroking the pedal as in manual bleeding (no power bleeder). It is possible that air is trapped between the two pistons in the M/C. (The booster push rod moves the rear piston directly, but the front piston is moved hydraulicly via fluid captured between the pistons.)

There is also the possibility that one of the cups on the M/C pistons will not hold any pressure, and that fluid is bypassing internally and returning to the reservoir.

[Chuckler3]

how would detaching one of the three lines free up that air, if there is any, between the two pistons in the MC?

Not being snippy. Actually curious as I thought I had an understanding of braking systems. That is, until this car came along and now I feel like I've been mislead my entire life.

[Chuckler3]

In what is probably an unrelated situation, there is also a wheezing noise (think about an 80 year old lifetime smoker trying to whistle) coming from the front driver's side of the motor when the brakes are being pump to check pedal feel.

[Frank Reiner]

Re: M/C bleeding

Cracking a line at the M/C serves the same function as a bleeder screw if installed at the same place. When the line is loosened, fluid can be expelled, and with it air, by stroking the pedal. As with all manual bleeding, the line must be tightened before releasing the pedal. The pedal can then be released, fluid will be drawn into the cyl. from the reservoir, pressure can then be applied again to the pedal, the line opened, fluid and air expelled, etc.

The sequence can then be repeated for the other section of the M/C.
By performing full stroke bleeding of each section separately, you can be assured of eliminating virtually all air from the M/C.

As to the "wheezing", two possibilities:
1. You are hearing the check valve and/or the diaphragm of the booster.
2. You are hearing the diaphragm of the vacuum pump on the engine.

Those sounds should disappear after several strokes of the pedal, after engine shut down, i.e., when all vacuum has been exhausted. If they continue in a no vacuum situation, a damaged diaphragm is likely.

[dirtyharry]

I think the wheezing your describing is the vacuum pump at the front of the 617, its normal to hear a sound from it whenever you pump the brakes(engine off)?? i think as its trying to draw from the pump. It is possible for the rear seal on the master to leak into the booster as well, if you unbolt it you may find brake fluid in there. it could be that the previous owner never followed procedure as far as bleeding the master/ he destroyed the rubber on the pistons by stroking too much while bleeding. taking any of the lines off will not free up air, but bleeding it back into the cylinder will. or as Frank suggested cracking the lines.

[Chuckler3]

Originally Posted by dirtyharry

I think the wheezing your describing is the vacuum pump at the front of the 617, its normal to hear a sound from it whenever you pump the brakes(engine off)?? i think as its trying to draw from the pump. It is possible for the rear seal on the master to leak into the booster as well, if you unbolt it you may find brake fluid in there. it could be that the previous owner never followed procedure as far as bleeding the master/ he destroyed the rubber on the pistons by stroking too much while bleeding. taking any of the lines off will not free up air, but bleeding it back into the cylinder will. or as Frank suggested cracking the lines.

Thanks for the response. What exactly do you mean by bleed it back into the cylinder? I've heard of leaking into the booster before, but I'm kind of lost with the rest of your post.

[Chuckler3]

So it was the master cylinder. There was a "ball" missing in the old one as seen in the photos below.

New


Old


Now I have to figure out this white smoke... I say this because I'm thinking it's bad rings, but I don't care to believe it. May be looking for a donor motor if this is the case...

[dirtyharry]

white smoke could be oil being pulled from vacuum pump into engine.. more likely is poor combustion caused by dirty or bad injectors or injection pump, you should do a compression test either way.

here are some images

OM617 Vacuum pumps

"Regarding the vac pump, look for engine oil in the large vac line that leads to the intake manifold. Its a diaphragm inside the pump that cracks and wears out and then engine oil gets sucked into the manifold vac line. Rebuild kits include the diaphragm, gaskets etc. But having the engine shut down vacuum solenoid working is very good sign, that indicates the vac pump diaphragm may not be dead. "

as far as bleeding back into the master, you make short strokes with a bleeder kit which should be included with the master when you got it... or available at any auto store, two or three plastic fittings with hoses run back into the master will push fluid and air, returning them to the top of reservoir. you should have some instructions or should be able to find them on the net some place.