brake problem

[SkyRyder55]

So my 2003 wagon seems to be having a brake issue. If the engine is off and i press the brakes the pedal is firm and does not sink at all.
Once i start it up, the pedal is soft and will slowly go almost to the floor. If i pump the brakes a few times it will firm up the pedal, but will still sink slowly.
Brakes have been bled.
Thoughts on this?

 

[UhOh]

Vacuum leak? Check main vacuum hose to booster. Hopefully it's not a bad booster.

When this this first appear as a problem?

 

[SkyRyder55]

Don't really know. Wagon is new to me. I did engine swap, replaced all pads and rotors. Never really drove it before engine swap.
I have another booster from parts car. How would I test the booster?

 

[Corsair]

My 0.02-
A vacuum leak causes loss of function of the brake booster, resulting in a brake pedal that feels firm, not soft. (ie. I don't think a vacuum leak is the issue in this case).

Soft / sinking pedal is most often an indicator of air in the system somewhere or possibly a failing master cylinder. I'm not sure how to test a master cylinder out of the car.

 

[SkyRyder55]

But would the problem still not be present then with out the car running? If master cylinder is failed it should still sink without vacuum assist.

 

[Vince Waldon]

Brakes sinking slowly to the floor is generally a leak somewhere or a master cylinder issue... adding vacuum assist by turning the engine on generally just makes it very obvious, particularly if it's a seal that only fails under pressure.

You don't mention if there's any fluid loss associated with this? Otherwise, my money's on a defective seal in the MC that's letting fluid bleed thru from one section to the other. Pretty typical failure mode, particularly if there's no fluid being lost.

 

[SkyRyder55]

no loss of fluid noted. I was jamming a 2x4 between brake pedal and seat base to hold wheels when doing axle nuts. There was no loss of clamping force after it sat there for hours.
I guess ill try pinching off all the brake lines to isolate the issue to abs/MC unit.

 

[Vince Waldon]

I was jamming a 2x4 between brake pedal and seat base to hold wheels when doing axle nuts. There was no loss of clamping force after it sat there for hours.

Hmmm... yup, that is a strange one then.

Did this behaviour start *after* you did the axle nuts?

 

[SkyRyder55]

I don't really know. I bought the car as a non running unit. Drove it home. Swapped in new engine. Confirmed drive ability. then did brake work. So I never noticed the brake issue till now. as I've gotten temp insurance to get the safety done.