brake issues, help needed

[jasper110]

i'm looking for some help with the brakes on my '85 v8 110. when applying the brakes the pedals firm, but if i brake for a longer period the pedal then goes soft as if there is air in the system. if i remove my foot from the pedal and re-apply it, the pedal goes firm again. this can happen when driving or even if sitting with my foot on the brakes at say a road junction. i've had the brakes bled twice by the local garage and they say all is well. i also have stainless flexi pipes, so it's not those. i'm quite happy to replace parts, and was even thinking of doing the calipers as they're quite long in the tooth now. however i'm not sure where to start. can you offer some advice?

[abaddonxi]

That sounds like vacuum loss.

From JDNSW in this (http://www.aulro.com/afvb/showthread.php't=29896&highlight=brake+vacuum+test) thread.



4. Check that the vacuum pump is working - apply the brake hard several times while stopped with the engine off, to use any remaining vacuum, then hold the brake pedal hard down. When you start the engine, the pedal should move down as vacuum becomes available. If it does not you could have a failed vacuum pump, a vacuum leak either in the line or the booster or a blocked vent on the booster.

Cheers
Simon

[DEFENDERZOOK]

are you saying that if you keep your foot on the brakes the pedal slowly sinks to floor after a while.....?


if this is what you mean it sounds like your master cylinder is shot.......

[JDNSW]

Between the two they have pretty much covered it. As I understand the symptoms, the pedal is going down when you hold your foot on it - this will be because vacuum is building up (but it should not have to build up, leak or sick pump or booster or blocked vent) or because fluid is leaking past the primary seal on the master cylinder.
The only other possibility is a fluid leak somewhere, but I assume that you would have found this by now, although it may not have shown up (yet) if it is a rear wheel cylinder, but the first thing to do would be to carefully check for any leaks, anywhere. Last (only) time I had this symptom in my 110 it was a hole rubbed in a brake line below the driver's feet where the fuel line touched the brake line. (contrary to what you might think, a nylon fuel line touching a steel brake line will rub through the steel line first - dust and grit embeds in the softer material and rubs against the harder material like a file)

John

[jasper110]

are you saying that if you keep your foot on the brakes the pedal slowly sinks to floor after a while.....?


if this is what you mean it sounds like your master cylinder is shot.......


that is exactly what i'm saying. thanks.

[BigJon]

I concur, master cylinder is bypassing internally. Repair right now!!! Don't wait, because the next time you use the brakes might be the time it fails completely:eek: .

I have some experience of this happening to a customer in the past (she claimed she couldn't afford the repair and asked if she could take the car home. I told her to stick to back streets and drive slowly, park it and don't drive it until it is repaired. Got a phone call that afternoon, she had rear ended another vehicle on the far side of town - No Brakes!. Stupid woman...:p ).

[DEFENDERZOOK]

hand brakes are a good idea in such emergencies......