Strange brake issue - firm pedal collapses

[jamlip]

Car is a 1968 912 with 911SC front calipers, 912 rears and a 19mm dual-circuit master cylinder.

I have just fitted a new (ATE) master cylinder as I suspected the old one was failing. The system has been bled multiple times - I’ve put perhaps a gallon of fluid through the lines!

I can get a very good, firm brake pedal, which holds. However, if I release pressure just a touch, the pedal slowly collapses (although it never reaches the floor).

The way this translates in driving the car is that the brake pedal has initial normal, firm feel, but after three seconds or so it slowly gives way.

I’m absolutely stumped - have never experienced anything like this, and I can’t visualize what’s happened here.

Would love to hear from anyone with previous experience of this issue!

James

[Superman]

If a brake pedal works fine when you stab it, but will go to the floor if you press gently, then you have a master cylinder problem. The seals are letting fluid past them, when you press gently. They should seal instead. What happens when you stab the brakes is that you create enough pressure to force the seal against the piston, and if the force is maintained the seal is holding that pressure.

Or maybe those are not your symptoms?

[jamlip]

Yes, that's absolutely what's going on.

Ugh. Brand new ATE part, too. Hopefully Pelican will be quick to replace it.

And it's such a pleasure to lie under the car withe brake fluid running down your arms. Cannot wait to do it again

Thanks for the help!

James

[winders]

Or a broken hub….

[jbs3]

Before you remove a new master i would check all the lines to each wheel and their respective calipers. I had the exact same issue on a truck i owned, including buying a new master cylinder only to eventually finding a leak on one of the brake lines which would hold on initial brake pedal press but slowly leak after and have pedal go to floor.

[jamlip]

Thanks. Should have mentioned - no fluid loss anywhere. Also checked every joint in the system.