76 (non-brake booster) 911s brake pedal push rod issue driving me nuts!!! please help

[filluptieu]

Hello guys,

I have a 997 and just bought a 76 Targa 911s!

So far, I'm learning a ton and enjoying it however this brake issue is starting to drive me nuts...

The problem: The brake pedal travels roughly 50% of the distance before I feel any resistance/pressure from the master cylinder. (I understand I do not have a brake booster so the operation requires a bit more muscle to engage the brakes however after reading multiple post on non-boosted 911's it seems as if I am using a bit too much to ensure a safe stop. Some times I have to lift my feet off the ground to really give it some force. Also with the far travel before I feel like the MC is engaging, I tend to begin brake 2-3 seconds later which causes some stress on my part and feeling as if the car isnt going to stop quick enough.)

My analysis and work to attempt to resolve the issue: I initially thought maybe the system needs to be bleed, but after bleeding the brakes about 8 rounds, the pedal travel only reduced fractionally.
Then I attempted to adjust the MC push rod that connects to the pedal and this resolved the problem however when the adjustment was right, I begin to fear there is not enough thread on the push rod. I decided to have the threads about 50% of the way down on the sleeve to at least have a few more threads utilized on the rod (I tried to get that in the pictures I attached). After looking into the workshop manual, there is no information on the pre-brake boosted setup to detail how to correctly set the push rod distance. By my inspection, this push rod and push rod sleeve is the correct one for my vehicle and looks original.




Without an instructions, I wondered if I had the incorrect MC however upon close inspection, the MC looks to be recently replaced as it was in great condition and per the seller, this MC was replaced. Then I thought, what if the MC is a knock off and has a piston issue, however I was able to verify the stamp on the top of the MC in the cabin which has the "ATE" stamp and this I believe is OEM.




Where am I at now: I'm completely at the end of my tracks... The only this I can think of is to max out the threads on the push rod to remove the gap in the pedal travel and in return, allow me to engage the brake operations sooner than further down in the pedal travel. Again, my concern is that the the push rod has such few threads that with the force applied by my foot, it could cause damage and strip the threads. Is it possible that the piston in the MC is not releasing or is there still air in the MC itself? I know for other cars I have built, its a lot easier to bench bleed an MC before installing. Could this be the problem?

[Bill Verburg]

Originally Posted by filluptieu

Hello guys,

I have a 997 and just bought a 76 Targa 911s!

So far, I'm learning a ton and enjoying it however this brake issue is starting to drive me nuts...

The problem: The brake pedal travels roughly 50% of the distance before I feel any resistance/pressure from the master cylinder. (I understand I do not have a brake booster so the operation requires a bit more muscle to engage the brakes however after reading multiple post on non-boosted 911's it seems as if I am using a bit too much to ensure a safe stop. Some times I have to lift my feet off the ground to really give it some force. Also with the far travel before I feel like the MC is engaging, I tend to begin brake 2-3 seconds later which causes some stress on my part and feeling as if the car isnt going to stop quick enough.)

My analysis and work to attempt to resolve the issue: I initially thought maybe the system needs to be bleed, but after bleeding the brakes about 8 rounds, the pedal travel only reduced fractionally.
Then I attempted to adjust the MC push rod that connects to the pedal and this resolved the problem however when the adjustment was right, I begin to fear there is not enough thread on the push rod. I decided to have the threads about 50% of the way down on the sleeve to at least have a few more threads utilized on the rod (I tried to get that in the pictures I attached). After looking into the workshop manual, there is no information on the pre-brake boosted setup to detail how to correctly set the push rod distance. By my inspection, this push rod and push rod sleeve is the correct one for my vehicle and looks original.




Without an instructions, I wondered if I had the incorrect MC however upon close inspection, the MC looks to be recently replaced as it was in great condition and per the seller, this MC was replaced. Then I thought, what if the MC is a knock off and has a piston issue, however I was able to verify the stamp on the top of the MC in the cabin which has the "ATE" stamp and this I believe is OEM.




Where am I at now: I'm completely at the end of my tracks... The only this I can think of is to max out the threads on the push rod to remove the gap in the pedal travel and in return, allow me to engage the brake operations sooner than further down in the pedal travel. Again, my concern is that the the push rod has such few threads that with the force applied by my foot, it could cause damage and strip the threads. Is it possible that the piston in the MC is not releasing or is there still air in the MC itself? I know for other cars I have built, its a lot easier to bench bleed an MC before installing. Could this be the problem?

Something is very wrong, you should have a solid pedal @ 1mm + a small amount

spec for play is 1mm

This is what the internals look like, plus a few springs


schematic of a 19mm ATE



The lower one here is a stock 19mm for a '76 911


Best guess from here is an internal failure, though I suppose it's possible someone put the wrong rod in there or a massive amount of air in the system



have the calipers at each wheel been bled?

[filluptieu]

Yes, I did bleed the brakes at all 4 corners.

[OSC911]

I had a similat issue when i replaced my M/C on my '76 coupe. I just couldn't get a good pedal. I could only manage 50% travel no matter how many times i bled the brakes. I tried the gravity bleed method on each caliper for about 10 minutes each corner and my pedal travel reduced to perfect height. I then just gave the pushrod 1-2mm free travel between it and the piston and it isnow great. Hope this helps.

[Bill Verburg]

When the brakes hit the hard spot of the pedal travel
are all 4 calipers engaged?
only the front?
only the rear?

[filluptieu]

Originally Posted by Bill Verburg

When the brakes hit the hard spot of the pedal travel
are all 4 calipers engaged?
only the front?
only the rear?

Not sure. I'll try it but what does this tell us? That the MC is not bled properly?

[filluptieu]

Originally Posted by OSC911

I had a similat issue when i replaced my M/C on my '76 coupe. I just couldn't get a good pedal. I could only manage 50% travel no matter how many times i bled the brakes. I tried the gravity bleed method on each caliper for about 10 minutes each corner and my pedal travel reduced to perfect height. I then just gave the pushrod 1-2mm free travel between it and the piston and it isnow great. Hope this helps.

It's possible that maybe the push rod was not set with enough distance to begin and as I bled the brakes the piston in the MC was unable to have full travel distance.

[911pcars]

Assuming the front and rear brakes apply adequate force to the brake pads, the master cylinder was installed correctly, the system operates to spec and your pedal has enough travel in full lockup mode, the issue might boil down to a shorter than needed push rod.

If that's all, the solution is to make the push rod long enough to adjust and have full thread engagement, yes? While it appears the push rod is fully locked in and not going anywhere, it looks like another 4 threads of bolt length will comfortably fill that thread area in the brake pedal pivot piece.



Sherwood

[Bill Verburg]

Originally Posted by filluptieu

Not sure. I'll try it but what does this tell us? That the MC is not bled properly?

each of the pistons I showed in the above post activates 1 of 2 circuits
the port/piston nearest the driver is the front circuit
the port/piston furthest from the driver is the rear circuit

If all 4 calipers are functioning after the long travel then it's likely that there is air probably in the m/c since the calipers were bled,, did you use a pressure bleeder?

if only 1 of the circuits is functioning that indicates an issue in the other