Thread: Hard brake pedal in reverse

Originally Posted by PerthDisco

The rock hard pressing steel is run out of boost which to be fair all cars do if you continually pump the pedal but on a 4wd it’s hard to use engine braking when reversing a boat and trailer down the boat ramp.

Maybe it’s just a natural quirk. When our last booster failed it when to full time pressing steel feeling.

Yes and no. When the booster fails (or runs out of vacuum) the pedal piston connects with the master cylinder piston. At that point you are pushing directly on the MC and with enough force you can still activate the brakes (both feet on the pedal and pulling on the steering wheel kind of force). In this instance there was nothing left. I know I can still brake the car when the vacuum is exhausted as I've tried it when I was going through the HPFP issues. This was different. With the benefit of hindsight I should have hit the EPB because at the speed I was going it would have fired up the pump to brake rather than tried to apply the hand-brake shoes.

I'm budgeting for a booster and master cylinder. The first time I drained the booster I reckon there was enough oil in there to have contaminated the seals on the MC.

The issue as I see it is the vapor pressure of the oil. When there is significant vacuum, the oil will travel past the non-return valves even when pulling from the booster to the pump. That oil will then condense in the booster. They tried to fix it with an additional NRV, but I don't think there's a way around it. I thought about a "catch can", but the problem is the oil vapor under vacuum. It'd just pass through the bottle. An electric vacuum pump is the final answer, but coming up with a solution that is affordable and available is another story entirely.

Originally Posted by BradC

Yes and no. When the booster fails (or runs out of vacuum) the pedal piston connects with the master cylinder piston. At that point you are pushing directly on the MC and with enough force you can still activate the brakes (both feet on the pedal and pulling on the steering wheel kind of force). In this instance there was nothing left. I know I can still brake the car when the vacuum is exhausted as I've tried it when I was going through the HPFP issues. This was different. With the benefit of hindsight I should have hit the EPB because at the speed I was going it would have fired up the pump to brake rather than tried to apply the hand-brake shoes.

I'm budgeting for a booster and master cylinder. The first time I drained the booster I reckon there was enough oil in there to have contaminated the seals on the MC.

The issue as I see it is the vapor pressure of the oil. When there is significant vacuum, the oil will travel past the non-return valves even when pulling from the booster to the pump. That oil will then condense in the booster. They tried to fix it with an additional NRV, but I don't think there's a way around it. I thought about a "catch can", but the problem is the oil vapor under vacuum. It'd just pass through the bottle. An electric vacuum pump is the final answer, but coming up with a solution that is affordable and available is another story entirely.

I agree on the vapour as there is no appreciable oil in the pipes when you undo the quick disconnect