Air in Brake System?

Paul6B

New Member
#1
Help needed please. I put a post on here a few weeks ago about air in the single line brake system on my '72 V8. After bleeding the rear calipers (big air bubbles came out). Talking to quite a few people, it seemed to boil down to the master cylinder, so I replaced it with a new one. I still have spongy brakes with no fluid loss. If I pump them the pedal comes back. Could this be the servo? Its now doing my head in. Got some time to think about it as the car is in having an auto gearbox overhaul and remote gearbox oil cooler fitted over the next couple of days lol..thanks, Paul. Still love it though...
 

harveyp6

Well-Known Member
#2
As a first step I'd clamp the flexible hoses to see if that improves the pedal. If it does you still have air in the calipers, if not, you have eliminated them, and can move on to the reast of the system. Just because you don't have fluid loss, it doesn't mean that air isn't getting in to the system somewhere.
 

Paul6B

New Member
#3
As a first step I'd clamp the flexible hoses to see if that improves the pedal. If it does you still have air in the calipers, if not, you have eliminated them, and can move on to the reast of the system. Just because you don't have fluid loss, it doesn't mean that air isn't getting in to the system somewhere.
Thanks Harvey. I understand that I am still getting air into the brake system. I have bled both rear calipers three times and got air out and the pedal back, so not sure what to do now.
 

harveyp6

Well-Known Member
#6
You should bleed the rear before the fronts, so if you get air out at the rear and that revives the pedal, and then no air comes out of the front, then I would think that it's drawing air into the line to the rears. Between the brake light switch union and the rear calipers.
 

Paul6B

New Member
#7
You should bleed the rear before the fronts, so if you get air out at the rear and that revives the pedal, and then no air comes out of the front, then I would think that it's drawing air into the line to the rears. Between the brake light switch union and the rear calipers.
Thanks Harvey. The cars in having its auto box overhauled and a remote oil cooler fitted after it overheated and spat all of its fluid out on the M40, so I'll try again when I get if back. My middle name is not lucky ag the moment. Cheers, Paul.
 

roverp480

Active Member
#10
What is not clear to me is whether the pedal is hard if the servo is evacuated by pumping the pedal, without the engine running. If so, its the servo that's introducing the soft pedal, not air in the system.
 

harveyp6

Well-Known Member
#11
What is not clear to me is whether the pedal is hard if the servo is evacuated by pumping the pedal, without the engine running. If so, its the servo that's introducing the soft pedal, not air in the system.
Pumping the pedal, whether the engine is running or not, won't prove for certain that it's the servo at fault. Plus add to that the fact that there is air getting into the system, it's not just a soft pedal. It has been bled multiple times, and there is always air being bled out, so it must be getting in there somehow. As an aside, it is possible to make up a hard pipe to bypass the servo to eliminate it that way.
 
#12
What is not clear to me is whether the pedal is hard if the servo is evacuated by pumping the pedal, without the engine running. If so, its the servo that's introducing the soft pedal, not air in the system.
Thanks Harvey, I'll try that when I get the car back...
 

clive P62

Active Member
#15
Might be worth checking the rubber supply pipe from resevoir to master cylinder.
I've seen many loose fitting ones due to age and can draw air in from there.
 

roverp480

Active Member
#17
I didn't pick up the fact the system had been bled a number of times, each expelling air , so as Harvey states it must be getting in somewhere. Having said that, my brakes on my LR Discovery 3 have a soft feel under servo conditions, ie engine running, but are rock hard without servo assistance., and I wondered if yours were the same.
 
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