engine revs up when pressing brake pedal and tiny air bubbles in fluid

[jcbadger]

Hi All

I have just rebuild all 4 calipers with new seals so know they are not leaking and having problems with tiny air bubbles when bleeding. I've bled many brakes before and have never had problems like this. There is a constant stream of tiny air bubbles in the bleed tube. I am using the old fashioned way of bleeding with tube attached to bleed screw and the other end in a jar. I have the engine running when bleeding so the servo is assisting with pumping the brakes. I've also noticed that the engine revs up (car in neutral) when I pump the brake pedal. This leads me to think that the engine reving up when I press the pedal and the constant tiny air bubbles are related - maybe the servo is faulty and causing both symptoms. Anyone got any thoughts? thanks

[aw11rally]

Sounds servo related to me too. I've never heard of one failing though.

Have you tried bleeding with the engine off? Any difference?

[jcbadger]

I've checked it again and the rev counter doesn't move up but there is a slight change in engine note - which I guess would happen because the servo vacuum comes off the engine.

I did bleed the rear calipers with the engine on and off. I've done the same with the fronts but it niggles me that there's so much air coming through the tube. Although I've done all 4 calipers I did one at a time to prevent too much air getting into the system - and the brake hose was clamped too. I'm not losing fluid and the master cylinder and servo exterior is bone dry.

[BIGBOBO]

I recently bled my brakes (after upgrading them) and I had the same problem. (I did however use one of those vacuum operated bleeders).

It took a while to find the leak but it turned out to be the bleed screws themselves not sealing properly. I took them out, wrapped them in a thick layer of teflon tape (keep the hole at the bottom free) and put them back in. After that the problem was fixed.

[jcbadger]

hi. yes I had a hunch this might also be the case - air sucked in pass the threads or around the hose where it connects onto the bleed nipple. I'll give it another go and put a jubilee clip around the hose and smear vaseline around the bleed nipple to block air getting in.

[PW@Woodsport]

The rising engine revs is the idle up VSV doing its job, it reacts when an extra electrical load is put on the system, in this case your brake lights, the VSV opens and allows extra air to bypass the throttle body, in effect simulating you slightly pressing the throttle.

It's totally normal.

I've never bled brakes with engine running, there is no gain in doing that, the air bubbles you see are coming past the bleed nipple threads.

[jcbadger]

many thanks for the technical clarification. All sorted. thanks for input