Poor brake pedal response only when engine running

[disciple31]

I have a '90 Plymouth Sundance - 2.5 L - on which I just replaced the front brake pads and also replaced the left front caliper.

I bled the brakes and got an excellent, firm, short pedal.

When driving it the next day, the pedal response was spongy and, with constant pressure applied, the pedal would slowly go to the floor. Upon turning the engine off, the pedal response eventually becomes firm again.

I suspect it is a swollen brake line which only swells under the added pressure of the booster -- what is the best way to check for swollen brake lines?

Any other ideas of what could be wrong?

[ShelGame]

Vac leak at the booster?

[mpgmike]

You have either a leaking line (check to be sure you got the washer on correctly at the caliper where the rubber hose bolts on) or a bad master cylinder. Either way, check your brake fluid immediately. A bad master cylinder will sometimes leak into the power booster, causing it to fail. Remove the 2 nuts holding the MC in place and see if there is brake fluid leaking out the back (after you verify the brake hose was connected properly, of course).

Mike
www.PowreHaus.com

[turbovanman²]

4 things-bad caliper that you just put on, the other one has now gone bad, you should always replace in pairs, line is bad or what it really sounds like, bad MC. If you do replace the MC, buy a new one only, I can't stress this enough.
To check for spongy lines, start it up and you or someone watch the lines, have a buddy or you stand on the brake pedal, it will be easily noticed which one is bad.

[Turbodave]

I still wouldn't rule out air in the lines, is the bleeder on the top of the caliper? I've seen the parts stores mix up calipers and if you get the wrong side the bleeder will be in the wrong position making it impossible for you to get all the air out.

[Whorse]

Make sure the check valve on the brake booster is installed correctly and isn't leaking. I had a similar effect with my car using a leaky check valve.