Brakes soft after sitting a few years

[rball716]

Okay...so everyone's nightmare...Built the truck and it was great. Then I went fuel injection and had a major issue, bought a house, got married....truck went on the back burner for a few years.

The good news...I got it running again. It's an 81 shortbox and when I built it I replaced the mater cylinder, can't remember if I replaced the booster (but I replaced most things), new brake lines, new calipers in front and drums in back, brakes worked totally normal (normal enough to drag race it).

Now after sitting, The pedal goes almost to the floor before getting firm. Its not a "soft" pedal, its like pushing air out of a balloon for the first 3/4 of travel then firm. With the engine off it pumps up to firm higher up like you would expect.

I bled all 4 corners with new fluid and that seems fine. The fact that it pumps up firm with the engine off seems to indicate the booster is fine. Not sure what could be going on given the brakes were great before it was parked and nothing changed. My guess was the fluid, which is why i bled through all 4 corners.

[Dead Parrot]

Possible a small ring of rust happened in the master cylinder while it sat. First push on the brakes runs the nice new rubber seal over the rust, slightly damaging the seal. Even if the rust is now gone due to repeated use, the damage to the seal is done.

[rball716]

Interesting thought. This would be the internal seal causing internal leakage?

[Dead Parrot]

Yes, internal unseen leakage. Had something similar happen shortly after I bought a boat. While towing the boat, the brakes suddenly went soft after a couple of hard stops on the highway due to merging traffic. First trailer that truck had towed in over a decade. The hard stops pushed the seal into previously unused territory and it found rust. Replaced the MC and all was good thereafter.

If your MC is a step or two up from the normal plain parts store MC, might see if there is a rebuild kit. Probably a pass or two with a hone and a new set of seals will fix it.

[rball716]

Update: Bled the brakes multiple times again, both with a vacuum bleeder and by dragging my wife out. No change, no bubbles. Replaced the master cylinder with a Wilwood Tandem. Bench bled that, installed, and then re-bled all 4 corners. Now I had some small bubbles before I got a big air pocket on the passenger rear. The other 3 corners had small bubbles for a bit and then cleared up. Previously I had no bubbles through multiple hours and multiple bottles of fluid.

Brakes are back!

Did I need a Wilwood master cylinder? Probably not...Just a stubborn air pocket somewhere it seems, maybe the new cylinder let it move enough to escape. But it gave me a good excuse!