Replaced brake booster causes grown man to cry...

[acupunk]

Hey gang,

Ok...replaced the brake booster on my 1997 boxster yesterday... the old one was full of water and completely rusted. After putting everything back together and using the Motive pressure bleeder to bleed brakes, I took her out for a spin. It was braking and driving well for about a mile, THEN the car started to slow down as the brakes engaged without me depressing the pedal and the brake pedal became rock hard (no travel). I barely made it home because, I believe, the brakes were fully engaged.

What did I do wrong?

Thanks in advance!!! :)

[wvicary]

Measure the free play between the brake pedal and the master cylinder. Was it new parts you installed? I have also seen defective master cylinders cause this issue.

[tony_986]

You still have air in the line.

Seen this before..

[acupunk]

Thanks for the replies!

Two questions: how do I measure the free play between the pedal and master? and...

Will air in the line cause the brakes to "self" engage and a hard brake pedal? I had only heard of the spongy brake pedal for air in lines...

[wvicary]

I've not seen air cause this type of problem. You should be able to feel the free play in the pedal.

[tony_986]

I've had air stuck in the master cylinder between the reservoir intake and plunger.

Caused the brakes once engaged to stay applied..

Took the car out for a test and once the brakes were applied, they stayed engaged.

Re-bleed the brake lines and it corrected the issue.. It never happened again.

[wvicary]

I've had air stuck in the master cylinder between the reservoir intake and plunger.

Caused the brakes once engaged to stay applied..

Took the car out for a test and once the brakes were applied, they stayed engaged.

Re-bleed the brake lines and it corrected the issue.. It never happened again.

I don't understand how this could happen. Air is compressible and would lead to a soft pedal or excessive pedal travel but not keep the brake master engaged against it's internal spring pressure.

[tony_986]

I don't understand how this could happen. Air is compressible and would lead to a soft pedal or excessive pedal travel but not keep the brake master engaged against it's internal spring pressure.

I agree with you, I couldn't understand why air would cause the brakes to engage and not release. The only thing replaced on the car was the master cylinder.

After releasing the brake fluid pressure and re-bleeding the brakes it never happened again and the brakes worked perfectly afterwards..

[acupunk]

i appreciate the responses.

The brakes stayed completely locked with no play at pedal...even when adjusted.

If it ever stops raining here in Tennessee, I'll try bleeding the system again. If that doesn't work at least releasing the brake pressure at the bleeder valves will allow me to get it on a tow truck. :(

[acupunk]

wvicary, tony_986, and the gang...

I rebled the system to no avail (pressure bled 3 times, and then brake pedal pumped for a fourth time around). When adjusting the brake pedal...I think I was doing it wrong.

Would I be correct in assuming that the closer the nut (from the pedal side) goes toward the brake booster (on the bolt) the MORE play you would get at the pedal ?

I've been trying to do this but the nut is a bit stuck and when I try to tighten it, the bolt from the brake booster spins with it.

Is there a trick to getting the bolt to not spin during tightening?

Thanks

[leg911]

Curious how this issue was resolved ?