That's a nice article Roger. Just to be on the safe side I went to NAPA, Advance Auto and Auto Zone trying to buy a new prop valve and none of them carried it so I came home and started to put my old one back on. Out of the blue, a guy I met last night at the local cruisein showed up at my shop with a whole set of truble shooting instructions for "no brake pedal" and to my surprise he also had a brand new prop valve with him along with a switch centering tool. SOMEONE ABOVE is surley looking out for me. I was quite surprised because I barley knew him. Just goes to show you, car guys are the best. He is also a machinest and knows how to measure booster rod travel and everything associated with the pedal to m/c, so he volunteered to bring his indicators over and measure everything just to be sure before I put the system back together. With all new parts I couldn't have any more trouble ....... could I.

 

I installed the new prop valve and the guy I bought it from brought his mighty vac over and we bled and bled and bled the system again and again and could barly get any pedal. Today another buddy came over, ex racer/car builder, and again we gravity bled then pedal bled and bled, three more quarts of brake fluid. Finally we started to get pedal but we kept getting air out of the bleeders. We decided to replace the new chrome m/c with the original one that came on the car. After more bleeding we achieved a full hard pedal but as soon as we started the engine the pedal went to about half an inch off the floor. We bled several times more and would get a little air out of the rear calipers on the first bled each time gut no air from the front calipers. After we let the car sit for a few minutes we would again have a full hard pedal, start the engine and the pedal would go almost to the floor. Neither guy has ever experenced anything like this. I'm going to try to get a replacement for my new chrome m/c tomorrow because something is clearly wrong with the one I had on the car. We can't figue out why the pedal goes almost to the floor when the engine runs.

 

Is the pedal going to the floor by itself or when you push it with your foot?

 

I have been keeping an eye on this posting since I have been having the same problem with my '75, power brakes, manual, power steering, 350. After replacing hoses, hard lines, calipers, and bleeding the M/C and calipers I finally have hard pedal UNTIL I START THE CAR. At that point the pedal goes to the floor.
My prime suspect is the power boost canister, the black drum that the M/C attaches to. If you have PB, then I would suspect yours too. Vacuum enters the canister from the engine thru a check valve, and is to assist the brakes. It sounds like a leak somewhere, since you say the brakes come back after the car sits a while.
There is a check valve where the vacuum enters the canister. There are seals between the canister and the M/C, and the canister and firewall. Canister has a diaphragm inside. Any could be leaking and you loose vacuum. I think my problem is that the M/C push rod is out of adjustment. It has a screw at the end to make the rod longer. When I wound it out 4mm (longer) I have some pedal with the car running. I have not had a chance since to jack the car up to see if I have caused the brakes to drag excessively by doing this. If no drag then I may do another 2mm. I don't know if this helps, but it is the next place to look if you have PB.
Ed

 

I called Tuff Stuff and they are sending me a new chrome master clyinder tomorrow to replace the one I had on the car. I also have their brand new booster on the car. My m/c push rod is non adjustable, round on both ends. They assured me that I had the correct booster and m/c for the car. I'm not going to fool with it any more until I put the new replacement m/c on when I get it. I'm tired of spending money on brake fluid that that I bled off and through away. Has anyone else experinced the brake pedal going almost to the floor when you start the engine after having a full pedal prior to starting the engine? This is a new problem that I'm having after finally achieving a pedal, at least I seem to be making progress.

 

Pedal going to the floor after start up just means you still have air in the system.

I replaced a master cylinder on a 77 and bled it out today.

 

Well, I guess I have to bleed some more. I think I'll start buying the gallon size container of brake fluid. It looks like I'll be needing it. This car was a fun two year build. After finally completing the build this brake problem pops up and has taken the wind out of my sail.

 

Read up on holding the pin out on the dist block, to help bleeding.
Also find a spec on your stroke rod.
Chuck.

 

You didn't answer the question to post 63.
After starting the engine, did the pedal go to the floor all by itself, without touching the pedal with your foot?
Or after starting the engine, did you push the pedal to the floor with your foot?

These are two different problems with two different remedies.

 

Roger,
First I wanted to say I am not trying to hi-jack Billy's thread, just having a similar problem.
I read your post and thought I would try this;
Car not running = good hard pedal.
Car running = pedal goes to floor when pushed.
Car running but vacuum line removed from booster and plugged (car won't run well without the plug) = hard pedal.
This test took 3 minutes in my bathrobe and slippers. No cost.
I'm a total noobie, but to me that says booster problem, at least in my case. There are two systems acting together, one hydraulic and one vacuum. If the hydraulic works in all cases where there is no vacuum, then the problem should be on the vacuum booster side. Makes sense to me, but I could be wrong.
Ed
PS If I am wrong, then it a trip to the parts store for a gallon of brake fluid and keep at it.
Where do you find specs on the push rod. I can't find it in the GM FSM?

 

Did you try driving the car with this "hard" pedal ?

 

Roger,
No, not in my PJs. What difference would it make?
Ed

Sorry, that came off as kind of smart-assed. Can't drive it far, maybe 6 feet forward and back in the driveway. No seats and other important things to take it on the road. I just thought of it in the shower, the pedal is not quite as hard car running, vacuum not connected. Rats, more bleeding, I guess!

 

In answer to post #63, with engine running the pedal goes almost to floor when you apply pressure to the pedal (push the pedal with foot). Another mystery, as in EasyEd's post, with vacuum disconected from booster and full hard pedal with engine off ....... start engine vacuum to booster still disconected ...... pedal not as full. Why would engine running have anything to do with pedal with vacuum to booster disconected. Well, just thought of something. Maybe some residual pressue still in booster? The m/c rod is non adjustable on my car. When the brake pedal is barley pushed by hand say 1/8 inch, the push rod immeadiatly moves correspondingly.

 

I just tuned into this post. I fiddled with my Tuff Stuff master cylinder
for about two weeks before i bought another brand from Ecklers. A few hours later I was driving. End of story.
Jackie

 

Are the Tuff Stuff masters just chromed Delco's ?

 

Billy,
I don't think it can be residual pressure. With the vacuum line disconnected or with the engine off if you depress the pedal you can hear the hiss of escaping vacuum, which bleeds the booster of - pressure. I'll try it again this PM, but I'm pretty sure I emptied the booster in my experiment this AM. Could be wrong, hadn't had coffee yet. Hope you get yours fixed. Might give me a clue to my problem.
I may have to call the brother-in-laws in on this. They are the resident experts on all things automotive. Maybe we can get a good deal on gallons of fluid if we go in together. LOL. Hang in there. It is fixable.
Ed

 

Tuff stuff shipped new m/c today, will recieve it on Tuesday hopefully. I don't know what brand master they use. I was thinking, ..... maybe I could buy Rodger a plane ticket and limo service to my door step if I keep losing hair and sleep. Return flight could stop at Easy Eds. Sounds like a plan to me. Rodger ?

 

Sure,I'll get my bags packed.

 

It usually takes 2 or 3 pedal applications to empty the booster.
If you are sure that there is no air in the system, with the engine off, you should be able to push on the brake pedal with both feet, with all your strength and the pedal should not go down more than 3/4 of an inch. That means it is air free and the master is good.

If you start the engine with foot pressure on the pedal, then it should lower by about an inch. That is normal.

If the pedal goes down to the floor while depressing it with engine on, and the above tests are true then the booster is bad. The air valve inside is probably not sealing. The booster has enough power with good vacuum, to depress the pedal all the way to the floor, by deflecting brake system parts and ballooning the hoses.

 

So, what we need is a booster Guru. Billy's problem and mine diverge at this point. His booster is new and, unless he forgot the firewall gasket or didn't torque the bolts (No offense intended Billy) there should be no reason for it to leak. Unless it it a defective unit. Mine on the other hand is probably the original that came on the car. An ugly rusty thing.
How then to proceed without throwing a lot of wasted money at the problem? I have searched the forum and found little about booster problems.
Starting at the front, the check valve and gasket seem fine. There is no gasket between the M/C and booster except for a rubber washer that wraps around a copper (?) washer. Rubber side out. Could that be in backwards? I can't find that part in any catalog.
If the diaphragm(s) is shot, or the air valve is sticking, then it would mean replacing the booster. If the firewall seal is leaking, I might as well replace the booster too, since I would have to pull the unit to do the job. Sounds like a Mother to get to the nuts.
Last, there is a rear air filter, but the GM FSM only says it can be removed, shaken out, washed, dried and reinstalled. Do you have to pull the booster to do this?
So it sounds like I am replacing the booster. Last question and I will stop my hi-jack. Do I just replace the booster, or should I replace the master too as a matched set? From who?
Thanks,
Ed