380SL Brake intermitent failure-help!!

[johnrose]

Can anyone help please - I have a 1981 380 SL which has an intermitent brake failure problem.
The first time it happened the brake pedal simply would go straight to the floor but then after leaving the car for a few days all was fine and brakes worked normally-there was no loss of brake fluid.
The fault occurred again in the same way so I had a new brake master cylinder fitted and all appeared well until recently when the fault occured again - still no loss of brake fluid.
What do I do now? The brakes are working again but obviously there is still a problem - do I renew the slave cylinder? Or is there somethong else I should be looking at?
Any suggestions would be a great help - thank you!

 

[star]

check that non of the brakes are sticking, so boiling the fluid, also that the pads aren't sticking. check the breather hole in the cap on the master cyl.

 

[astradave81]

Has it got ABS? It could be the pump. Bleed all the brakes to check for air. Get 4 brake pipe clamps clamp each flexible brake hose. Re-check pedal, if it sinks down with all flexible pipes clamped then its probably the pump..
If the pedal is fine with the clamps on then release 1 at a time and keep checking the pedal to see which corner is causing the problem.

 

[star]

Has it got ABS? It could be the pump. Bleed all the brakes to check for air. Get 4 brake pipe clamps clamp each flexible brake hose. Re-check pedal, if it sinks down with all flexible pipes clamped then its probably the pump..
If the pedal is fine with the clamps on then release 1 at a time and keep checking the pedal to see which corner is causing the problem.

Very unlikely to be pump, unless its leaking or firing off all the time, sorry.

 

[astradave81]

Well at my garage we've had 2 ABS pumps that have given the same symtoms as a faulty brake master cylinder on 2 completley different cars, naturally we replaced the master cylinder first as its the cheapest part. On one car we even replaced the calipers aswell only to find we had the same problem. So as to weather its unlikely i'd say it could be a definate culprit!! It didn't show up on our diagnostic equipment either as the motor in the pump was fine. I'm not saying it is the problem but it sounds likely if the master cylinder has been replaced and all the brakes have been bled..

 

[television]

There is no reason as to why the master cylinder has failed again, nothing else that can do it

 

[wireman]

If the master cylinder is not fully returning to its rest position the side ports which top up the system from the reservoir will not open and the brake pedal will travel excessively.
I am not familiar with the pedal box assy on your car but it could be caused by the brake light switch holding the pedal slightly depressed occasionaly, on this site there is a tale of a sprinter with the operating rod (between the pedal and servo) sufficiently maladjusted (too long) to cause this very problem.