Occasional brake pedal squish - vacuum or master cylinder?

[Locker537]

The car in question is a 1979 911 SC.

The majority of the time, the brake pedal is hard and engages early in the travel.

Occasionally, the pedal will go soft and sink to the floor. This seems to only happen when coming to stop and disengaging the clutch, prior to shifting into neutral and then back into first. It does not happen every time. It seems to happen more frequently after I've been driving for an hour or more.

I am not losing any brake fluid. I've checked the pedal cluster and marked the fluid reservoir and watched the line.

I also gave a general inspection to the vacuum hose near the master cylinder and it appears free of cracks and the rubber is nice and pliable.

I flushed the fluid recently, and went through the bleeding process an extra couple times to ensure there was no air in the system anywhere. I believe the master cylinder is original or at least OE Porsche given the part number, but the booster has an ATE sticker on it. I can take pictures if that is helpful.

My understanding is that a sinking brake pedal could be caused by a failing master cylinder. I also understand the ATE master cylinders on the market were a crap shoot as of a couple years ago.

How can I further diagnose this issue?

[Rawknees'Turbo]

One thing you can do to check the master cylinder function is to press hard on the pedal while the engine is off, keep heavy pressure on it for a bit, and see if the pedal begins to sink to the floor - that would indicate that the cylinder's piston seal(s) is allowing excessive fluid by (if the pedal drops), as long as there are no leaks in the system, like you noted.

Intermittent soft pedal usually indicates air in the system, however . . .

[Locker537]

Originally Posted by Rawknees'Turbo

One thing you can do to check the master cylinder function is to press hard on the pedal while the engine is off, keep heavy pressure on it for a bit, and see if the pedal begins to sink to the floor - that would indicate that the cylinder's piston seal(s) is allowing excessive fluid by (if the pedal drops), as long as there are no leaks in the system, like you noted.

Intermittent soft pedal usually indicates air in the system, however . . .

That's easy enough to try.

Bleeding the system again is also easy enough as a next step.

[john walker's workshop]

Dragging brakes cause heat, vaporize fluid, which is compressable.

[r lane]

Check to see if all 4 wheels spin freely to elliminate boiled brake fluid possibility. The seals in the master cylinder can fail and swap fluid internally so you won't see evidence of a leak and the pedal will sink to the floor

[Robey5]

I am watching with great interest, as I recently noticed my brakes behaving similarly since I got her back on the road this spring.

I’ve bled the system more than a few times since the time that I overhauled the calipers and replaced the fluid...... and now; it seems that the pedal likes to get squishy occasionally like yours (same year, but may not be a related reason).

[Locker537]

Originally Posted by Rawknees'Turbo

One thing you can do to check the master cylinder function is to press hard on the pedal while the engine is off, keep heavy pressure on it for a bit, and see if the pedal begins to sink to the floor - that would indicate that the cylinder's piston seal(s) is allowing excessive fluid by (if the pedal drops), as long as there are no leaks in the system, like you noted.

Intermittent soft pedal usually indicates air in the system, however . . .

I did the pedal test and it remained solid.

I did a very brief inspection of each caliper and found a small amount of brake fluid on the driver's rear caliper. There is clearly a small leak. It's not enough to cause a puddle. Just enough to linger on the bottom of the caliper.

I will probably lift the car and inspect further this weekend. The calipers in question are from an 1985 Carrera. I can't remember if they are ATE or PMB, but I have records.

[Locker537]

I have not investigated further yet, but I have a question.

If I have to remove the bleeder screw or fully remove the caliper, what's the best way to prevent the brake system from loosing all fluid?

[RDM]

Push the pedal down and prop it in place. This prevents fluid from passing through the master cylinder.

I use a piece of lumber between the seat base and brake pedal.

[Locker537]

Originally Posted by RDM

Push the pedal down and prop it in place. This prevents fluid from passing through the master cylinder.

I use a piece of lumber between the seat base and brake pedal.

Thanks for the tip. I've been avoiding this project a bit...

Given the caliper is not dragging, I'm hopeful the leak is at the bleeder screw and a thorough inspection and cleaning solves it.

[Locker537]

I confirmed the leak is from the caliper itself, likely one of the pistons today.

Weighing my options of rebuilding, purchasing restored calipers, or purchasing new from Pelican.



(An aside, but the photo I have uploaded it not rendering...)