Vac system

[caldirun]

The brake pedal is going down quite fast with the engine running, is there a circuit diagram for the vac system or a step by step guide for finding the leak/pump problem and what should the vac reading be at x RMP?

[macquatic]

HERE from martin1810s website. (Thank you Martin, lots of excellent info on it)

[scotty33]

Sorry I don't know the vac figures, but think the fault may be on the hydraulic circuits? Surely a vac leak would make the pedal harder? e.g. I recently had a rear caliper apart due to sticking handbrake. I bled the caliper with engine off and had same symptoms you have. I then bled again with engine running and the fault cleared. Trapped air must have caused it, but could see a master cyl problem or leak giving similar symptoms?

[caldirun]

I have never heard of bleeding the brakes with the engine running, it should be done with it off so no servo assistance is available.

[scotty33]

I read somewhere that ABS sometimes causes it to be difficult to bleed rears with the engine off, running the engine is said to help. I had trouble bleeding a rear caliper so tried again with the engine running I was succesful 2nd time. Maybe luck, maybe something in it - who knows?

[caldirun]

I read somewhere that ABS sometimes causes it to be difficult to bleed rears with the engine off, running the engine is said to help. I had trouble bleeding a rear caliper so tried again with the engine running I was succesful 2nd time. Maybe luck, maybe something in it - who knows?

I do think you are on the button, I had stripped the rears and have now checked the vac system and the servo, no problems but still the sinking pedal. I do have a very small amount of soft travel without the engine running, I will give your advice a try, thanks a lot

[caldirun]

I do think you are on the button, I had stripped the rears and have now checked the vac system and the servo, no problems but still the sinking pedal. I do have a very small amount of soft travel without the engine running, I will give your advice a try, thanks a lot

Today I bled the brakes with the engine running, still the same problem, my vacuum system checks out fine, no leaks and loads of depression which holds when I press the brake pedal with the engine running. Is this symptomatic of a failing brake servo?

[scotty33]

Sorry, I can't see how it would be servo issue? lack of assist or unwanted assist, yes servo, but soft pedal, for me, must be hydraulics? Is it possible the fronts need bleeding?

[caldirun]

Sorry, I can't see how it would be servo issue? lack of assist or unwanted assist, yes servo, but soft pedal, for me, must be hydraulics? Is it possible the fronts need bleeding?

I bled all four callipers yesterday. The brakes are perfect without the engine running, press on pedal it moves very slightly then is solid and holds that position, now start the engine, the pedal descends going to the floor in approx 12 seconds. The only change is the servo assistance comes in.

[zollaf]

without the servo you are not compressing the air, which you do easily with the help of the servo. are there bleed screws on the master cylinder itslef. if there are try bleeding these as well.