Brake Bleeding -Why does pedal get soft after time?

[WB72]

There are a ton of bleeding posts. I have checked most of them, but my problem is more of a question of why is this happening rather than how to bleed brakes.

While I was rebuilding my brake system, I replaced my MC. Bench bled it prior to install.
Replaced all hard lines and rubber lines.
Replaced wheel cylinders and callipers.
I used a vacuum pump to bleed system, then found a bad flare to the rear so I repaired it and re bled lines.
I had a stiff pedal and I left them alone for a month or so.

Today I went to work on it again and pressed the pedal and it was soft and brake light went on. I know it is PV with the light, but what would cause the soft pedal with no visible leaks at all connections?

I am going to have to bleed them again, but want to fix problem at this time before I take the truck on the road for it’s maiden voyage.

[bloo]

It must have had air in one of the circuits after sitting.

Things I would do (in addition to looking all the connections over again):

1) look real close at the combination valve, make sure there isn't a tiny bit of fluid leaking, maybe by the wire connection.

2) peel back the caps on any wheel cylinders, make sure you don't have one with some fluid in it. Adjust up the rear brakes nice and tight while you are there.

3) look over the calipers real close for wetness.

4) pull back the master cylinder from the booster (if you have one) or firewall and look for leakage at the master cylinder piston.

5) check that you don't have slop in the front wheel bearings (if front disc brakes).

If you dont find anything wrong at any of these places, I'm out of ideas. It would just about have to be a master cylinder defect. A master cylinder can leak fluid back into itself. Also, verify you have the right cylinder for the type of brakes you have. Disc/drum, drum/drum, and disc/disc would all be different.

[WB72]

Originally Posted by bloo

It must have had air in one of the circuits after sitting.

Things I would do (in addition to looking all the connections over again):



5) check that you don't have slop in the front wheel bearings (if front disc brakes).

If you dont find anything wrong at any of these places, I'm out of ideas. It would just about have to be a master cylinder defect. A master cylinder can leak fluid back into itself. Also, verify you have the right cylinder for the type of brakes you have. Disc/drum, drum/drum, and disc/disc would all be different.

Bearings are fine, what would be the issue if they were loose.

Also the cylinder is a match to the old MC. Disc/Drum. Is there a way to check the MC for defects?

[bloo]

Really no way to check a MC other than to look for it leaking at the piston, but if theres no leak anywhere else the MC just about has to be it.

A wheel bearing adjusted too loose will knock back the caliper piston too far, and then it takes more fluid to make the pads touch. It causes a low pedal, and might even cause you to have to take a second stab at the brakes. I don't think the light would normally come on because of this, but Im not sure. I didn't really suspect this, but I didn't want to leave anything out.

[AussieinNC]

Remove the PV switch that controls the light....

Press the brake pedal several times to recenter the trip valve...

Maybe invest in a pressure bleeder to force the fluid thru the system...

If the master cylinder has a defect in the bore or port positioning, it may....allow air to seep back into the lines over time....

Maybe time to try another mc....

[Ironangel]

Heres one that that has blown many minds. The bleeders have terrible wheel cylinder and caliper thread fitting issues. Many are machined in China, the USA and or both. Understanding that air compresses and fluids do not, sometimes air can be sucked in right past those threads on the bleeders when they're open. You can sometimes see this using Mighty Vacs with clear vinyl tubing with the presents of endless tiny micro bubbles while pumping the Vacuum lever. Smear only the threads with a small amount of this stuff https://shop.advanceautoparts.com/p/...E&gclsrc=aw.ds
and "gravity bleed" each rear wheel cylinder independently...If the problem persists, do the front bleeders. If the problem persists, then air is either getting in past the cups, or caliper piston seals, or is trapped in the system at the MC and/or the PV. Tiny bubbles will gather to be come big bubbles over time...