On my 87 4Runner.
At a stoplight, brakes seem good. Strong pedal.
As I sit there, the pedal slowly goes to the floor.
Pump it again and strong pedal, then slowly to the floor again.

Thought it was the booster. Changed that last night.
Same problem.

I don't want to throw $$$ at it guessing what is wrong.

Maybe the master cylinder???
No fluid leaking on the wheels, and the reservoir is not loosing fluid.

Any other ideas?

 

check ur lspv (Load Sensing Proportion by-pass Valve), its on the rear left axle and connects via a pivotong rod to the frame rail above right rear wheel, there is a bleeder on it and 3 (i think) brake lines, i had a problem w/ mine not too long ago on my 88...

bleed ur system including lspv and go from there...

 

Hmmmm.....
I replaced that valve about 6 months ago. Replaced both wheel cyl. & calipers. Complete bleed including the lspv.
I can look under the truck again tonight to see if anything is amis.

 

It's probably your MC failing. Before you turn your truck on, how is the pedal? It should be stiff, and when you turn the key, the booster kicks in and pulls it down some. I just replaced the mc on my car, and the pedal is WAY harder when the car is off.

It's too late, but a master cylinder is cheaper than a booster.

And also, the MC can fail without leaking. It leaks internally. Now that the pedal has been pushed to the floor, it's more likely the mc will go bad if it isn't already.

 

I agree with DJ. Had that same issue on a couple of cars. Always turned out to be the MC.

 

Haha! aren't brakes fun!!!