Brake Bleeding Question

[colnago]

I have an F250 with a brake booster. I had to replace my proprtioning valve today, which meant bleeding the system. I wanted to take a test drive, so I started the truck, and the brake pedal went straight to the floor. I found some leaks, snugged things up, rebled the system, and went for my test drive. I think that I still have some air in the line, so I still have to deal with that.

But my basic question is, since I have a booster, should I bleed the system with the engine running, or not running, or does it matter?

Thanks,

Joseph

[TNIceWolf]

I myself dont think it really matters. I never had any problems bleeding brakes with the engine not running. The master cylinder itself is going to work the same either way. The only difference would be the effort required to push the brake pedal.

[Busboy]

You don't need the booster in operation to bleed the system. I would limit the number of times I max stroked the booster when bleeding. The old rubber diaphragm isn't used to flexing to that degree.

[colnago]

The old rubber diaphragm isn't used to flexing to that degree.

Is that why you're supposed to put a 2x4 on the floor when bleeding the brakes?

Joseph

[Busboy]

Never heard that bit of wisdom.

[colnago]

I kept running into references on google that said you shouldn't run the pedal all the way to the floor when bleeding, and this can be controlled by putting a 2x4 on the floorboard.

Joseph

[MadMike]

Is that why you're supposed to put a 2x4 on the floor when bleeding the brakes?

Joseph

The reason you place something under the brake pedal when bleeding(using the two man pedal pump method) is to prevent over stroking the MC and failing the piston seals.

When new, the MC has a smooth clean bore and you can readily bottom out the pistons in the cylinder, as there is no risk of damage to the seals.
With an older or used MC, the bore is only clean and smooth in the normal area of travel when the brake system is closed. Open the system the pedal can be floored, this pushes the pistons into the untraveled area which can become rough from corrosion and contamination. This will damage the seals on the pistons. Initially the MC may feel fine, but usually on an older system(especially one that has not been bled in some time) the MC will fail. Random pedal drop or the pedal floors with power assitance, but feels normal without power assit.

Personally, I prefer the vacuum pump method as this avoids the above problem and it's just far easier. Suck out all the old fluid from the MC, clean out any trash. Then fill the MC with fresh fluid.
If you can, purchase new bleeder nipple(s) for the front and rear. Using yellow gas pipe tape, place ~two wraps around the threads.
Remove the old bleeder nipple, install your new tapped nipple in for an air tight seal(don't bottom it out, just snug it for an air tight seal) and then begin vacuum bleeding. I never pull more than 15inHg as with older wheel cylinders it is possible to cause them to leak. Remove the new bleeder, install your original, move onto the next corner, retape nipple, and begin again. For me this reduces mess and time. I don't have to futz with cleaning a probably clogged original bleeder or taping it up all the while that corner is piddling out brake fluid all over the axle. Bleeding only takes me a leisurely ~15-20mins and I can do it alone. More beer for me.