Potential Brake Failure?, Loss of brake pedal position Options

[rbzymek]

I was hoping for some direction to correct a brake issue. While driving last week my right foot was resting on the brake pedal and then it started moving down! The brakes still worked but the pedal was way lower than design position. I pulled over immediately and had the car towed home. I installed a new master cylinder 7 years ago (about 13,000 miles) from 914Rubber (not uro). All four corners were rebuilt by PMB. New flex hoses and pads were also installed. Brakes have been working fine prior to this hiccup. Fluid resevior was full and no leaks detected.

So I suspect the M/C but are there any other possibilities? Thanks in advance!

[bkrantz]

If truly no leaks and no fluid loss it might likely be a problem in the MC.

[emerygt350]

The only other thing would be a sudden change in the rear brake adjusters which seems impossible.... What happens when you pull the parking brake? Does the pedal travel decrease?

[rbzymek]

The only other thing would be a sudden change in the rear brake adjusters which seems impossible.... What happens when you pull the parking brake? Does the pedal travel decrease?

The hand brake has no affect. It behaves normally as far as pedal travel. I plan to bleed the system and watch it closely.

[flipb]

The MC mounts to relatively thin metal that is known to fatigue over time. Maybe you punched the MC off its mounting point.

[Ninja]

Any aftermarket master cylinder is suspect...

914 rubber did not make the cylinders themselves (I believe).
They subbed the work out.

Try to find an ATE master. They have gotten expensive.

I own an auto repair shop and I DON'T install anything but OEM master cylinders.

If it's discontinued (happens a lot on 15+ year old cars) I decline the job now-a-days.

[brant]

Did you pull the pedal board and then pull the rubber boot on the plunger

That’s where the leak is usually

[914Sixer]

Sent PM

[fiacra]

Sounds like an MC internal seal failure. If you put steady pressure on the brake pedal does it hit what feels temporarily like an end point but then starts to slowly sink down? If so, my money is on an internal MC failure. In that case you may not see a leak, but it is worth pulling the pedal board to check as a leak there is a common point of failure and brake fluid will mess with your pedal cluster. Best to attend to that right away. Not trying to bad mouth a certain supplier, but the MC I got from them was part of a known batch of defective MCs with a poorly pressed in Welch plug. They failed to notify me, It failed within 100 miles of driving, and they never followed through on a promise to reimburse me for the cost of the part. I still buy from them, but I will never get an aftermarket MC from them again. If you got one from them and got seven years of use out of it then count yourself lucky and get an ATE MC this time. Worth the extra $100 in my opinion. (NB: that's what it cost me, but with today's stupidity around trade and other unnecessarily created global instabilities I'm not sure how much more they cost now.)

I see that Mark PM'd you, and that is likely about the 17mm German made MCs that he has available. I think that's a reasonable choice. The downside to the 19mm ATE MCs is that they require a fair amount of extra pedal pressure to engage the brakes. I'm used to it so it doesn't bother me, but there is a period of time where you have to adjust to it. Mark is an honest, reputable supplier and if you want a 17mm MC then get one from him. There are arguments to be made both ways, but I would trust anything he is providing. Good luck!