911T Brake mystery, soft first pedal, solid 2nd, but slow loss of pressure

[rxtrom]

UPDATE: 4/4/20

SO I ordered, received and installed a new MC bled the system using motive pressure bleeder 2x and partner system 2x. The pedal is firm on the first press now but still slowly loses pressure albeit a lot less than before. I had my daughter press the brake pedal while I looked at each caliper and the MC for leaks. I found both front S calipers leaking by the brake pads. I am guessing this is from the piston? These are fresh rebuilds from PMB.

Seems I had 2 issues before and solved 1. I guess I have to pull the pads out and inspect but not sure how to proceed until calling PMB Monday..



3/28/20 Good morning,

My Saturday is relegated to solving my brake issues but I am worried I may need a new MC.

Background:
Full restoration which is now road worthy, sort of.
1.Replaced reservoir and its feed lines to the MC.
2.Replaced all calipers with PMB rebuilds. S calipers in the front, std in the rear.
3. Replaced several hardlines with the exception of the ones from the MC or the through body.
4. Used the PMB supplied soft lines, I know some people use SS braid, but this will not help my loss of pressure.

Brake system refill and bleed:
-I initially pulled the fluid from the res to each wheel using a vacuum.
-Then pressure bled each wheel 2x with the motive system.
**Today I gravity bled the 2 front wheels while gently banging on the caliper and checked for any leaks at the MC or foot pedal. Found none. Some small particles and a couple of air bubbles came out of the nipple into the hose I was using for the bleed, passenger side.

Current Situation:
Initial push of the brake pedal is very soft, 2nd is firm but after 4-5 seconds with constant pressure the pedal will slowly go to the floor and I will lose brakes.

The MC is probably original to the car but looks like it has fresh rubber on the piston in the foot well from the prior owner.

Is my MC bad? Should I gravity bleed the rears?

Thank you in advance.
Rob

[NYNick]

Rob,

After rebuilding my original calipers in the 72 I had a heck of a time getting the brakes bled properly. Every time I would take the car out for a spin, the pedal would feel too soft to me. It never went to the floor like yours.

I must've bled them 5 times using a Motive and once with just my wife pumping the brakes. Actually, that got the best results, but it still wasn't satisfactory.

The last time I made sure there were zero leaks from my overflow tube, I removed the rubber gasket from the reservoir cap and used it inside the Motive cap to insure a proper seal, pumped the pedal 10 times before I bled each caliper and pumped the Motive back up to 20-25 psi before I bled each caliper.

This seemed to work for me.

Nick

[911pcars]

Suggest bleeding all corners. However, the pedal should be firm at some point before it hits the floor. I assume new pads? Sometimes new piston seals pull the caliper pistons back too far and thus increasing the rotor contact clearance.

While gravity bleeding, use light hammer taps to encourage random air bubbles to dislodge and escape. Escaping air bubbles can be made visible by attaching a length of PVC/nylon tubing to the open bleed screw, then supporting the tube upward.

Still NG? After all easy-to-perform diagnosis completed, I would suspect a bad pressure seal in the MC. Even after removal, can be bench tested to confirm with a vise or equivalent.

Sherwood

[rxtrom]

Thanks guys. I bled the brakes again (partner method) and found some air bubbles but the pedal still will not hold steady pressure. It eventually (slowly) goes to the floor. I had to order a new one, but in the mean time I guess I can drain and remove the old one to test and verify. I assume getting it in a vise to push the piston to see if it leaks is all I need?

[john walker's workshop]

Same issue on my '69 after a new MC. Second new one fixed it. Annoying.

[911pcars]

Originally Posted by rxtrom

Thanks guys. I bled the brakes again (partner method) and found some air bubbles but the pedal still will not hold steady pressure. It eventually (slowly) goes to the floor. I had to order a new one, but in the mean time I guess I can drain and remove the old one to test and verify. I assume getting it in a vise to push the piston to see if it leaks is all I need?

To bench test, need to plug both outlets to confirm the internal seals are OK or NG.

[Arne2]

I used to do brakes for a living. The slow and steady pressure drop is a classic symptom of a bad master cylinder. I'd get a new one.

[rxtrom]

Thanks everyone. Replaced the MC still have an issue see update in first post.

[Tyson Schmidt]

I had leaky S calipers that were fresh from PMB as well.

Took them apart and found the groove where the piston seals sit was badly pitted. Not sure if that was caused by acid bath during the playing process or if they were pitted before. Sure looked like acid damage.

Had the same symptoms as you’re having.

[rxtrom]

Originally Posted by Tyson Schmidt

I had leaky S calipers that were fresh from PMB as well.

Took them apart and found the groove where the piston seals sit was badly pitted. Not sure if that was caused by acid bath during the playing process or if they were pitted before. Sure looked like acid damage.

Had the same symptoms as you’re having.

Interesting. I assume you were able to ship them to PMB and the fixed it?

[Tyson Schmidt]

No, I haven’t sent them back yet.
I replaced them with A calipers and contemplating what to do. Pretty sure these are now scrap metal.

[911pcars]

Tyson,
There used to be a repair (White Industries?) to sleeve a scored piston cylinder, yes?

Sherwood

[73pcar]

Sorry for your continued troubles and frustration. Odd that you are just now getting leaks at the caliper pistons which you did not experience prior to the new M/C and Motive bleed. Assume you used the recommend pressure on the Motive bottle. So, let me tell you about my PMB experience. I had the same impossible to bleed/good firm pedal after my 70 911 calipers (front and rear) were rebuilt by them. Having just been rebuilt - particularly by PMB - the calipers were the last thing I attributed symptoms to but they indeed were the issue. When they media blasted the calipers, they used - or may still use - very small plastic BB's - which were not fully removed during the-reassembly process. When I removed each bleeder screw (have no idea why I did that now, but glad I did), I found one or two these in the bleeder screw cavity...problem found. I don't recall now if I took the piston assemblies apart as well and found them, or the problem was just in the bleeder screw cavities. Anyway, simply enough to remove each bleeder and take a look. Good luck!