Brake loss after 15 minutes

[faiz]

Hey guys,

So my brakes are fine when I start up the car and drive for about 15 minutes.
Once the engines warm I can push the car a bit and the brakes are perfect.
Good solid braking, i can brake much later than my Impreza and theres no pull.

However, after about 15 minutes the brakes suddenly disappear and I find the pedal goes all the way to the floor until I pump once and they come back.
The longer I drive it means that I might be pumping twice before braking.

I have checked with my local mechanic quickly and there are no leaks and the reservoir is full.

I thought it could be caused by the master cylinder rubber going straight through the fluid but I am not sure that that is the case.
It has happened to me before and I know that after pumping the pedal will slowly go back to the floor under constant pressure.
In this case the pedal will maintain its position and not go back to the floor after pumping.

Seems really strange to me and I'm not sure where to start really.

Any suggestions or previous experience would be greatly appreciated!

[wasserkuhl]

OK.

What type of 911 is it?

I would be checking the brake fluid first, how old is it?When was the last time they were bled of air locks etc....

Old Fluid can cause the symptoms you describe.

Secondly the master cylinder may have failed, letting fluid past the seals and therefore dropping system pressure..............If your car has a booster or servo on it , this too may have failed and needs replacing..........

Thirdly, one or several of the brake lines may have ruptured with age.

A good mechanic will be able to diagnose and repair this problem.

[faiz]

Sorry I should have specified.
It's an 84 Carrera 3.2
My friend also suggested changing the fluid, I recently bought the car and imported it to Uganda so I honestly couldn't say how old the fluid is but I think i'll start with that first.

I'm fairly certain the rest of the system is ok as we removed the calipers and expected everything.
I'll call the mechanic and bleed the brakes and update you guys.

Thanks alot Wasserkuhl.
Pelican to the rescue as always.

[john walker's workshop]

dragging caliper(s) cause friction heat that can vaporize fluid. do you have that on any wheel? do the brakes come back after not driving it for a short while?