Okay, truck is an 85 4x4 4runner, i'm posting this here because the brakes are essentially the same as the 86-95 and there's more traffic

So, this all started with a bad booster and a mysterious loss of brake fluid. I replaced the booster and figured the master was leaking into the booster (since I couldn't find a leak) so I replaced that too. All seemed well until a big road trip to Moab. Halfway through it the pedal started needing a pump to get brakes. It would be good for a minute or two afterward, but then need a pump again.

My only guess at the time was my rear pads being maladjusted (no parking brake = no auto-adjust) so I adjusted them out with the star adjuster. No change other than the pedal was much higher after they started to grab.

Next guess was a bad bleed. I picked up some speed bleeders, and in the process of doing that noticed a leaky caliper. Replaced both calipers, got a great bleed including the LPSV, same problem.

So, next guess was a brake line expanding. Put some Marlin braided lines on, same problem.

The brakes feel better than ever after I pump them. And since the truck is an old 85 I am happy to have replaced everything. But I need this fixed, and I'm not sure where else to look.

The rotors and drums don't seem to be warped, at least not visibly at all while spinning. I would thing they would be pretty bad to push a caliper or wheel cylinder back in (which would cause a noticeable shimmy, and the need to pump up the pedal.)

Also, I need to do more testing, but it seems like I need to be driving to cause the issue. If the truck sits there idling for a few minutes, the brake pedal will still be in a good spot.

Thoughts? I was thinking perhaps I got a bad master, which isn't unheard of, but I don't know why that would only present itself while I drive.

 

"Pedal to the floor" is the classic sign of a bad master cylinder. The cylinder leaks fluid past the seals, but slowly enough that you can "outrun" it by pumping.

You have lots of symptoms, but the main one points to the MC.

 

Hmm, so a couple things are making me think it's not the master. First, the pedal does not slowly sink. If I sit there with a bunch of pressure on it it is solid for as long as my foot is on it. Second, if the truck is just idling for a while, or even when I park it overnight, the pedal does not need a pump right away. It is only once I start driving.

I have noticed a bit of play in my front right wheel bearing. Could that cause enough play to push a piston in a bit? Regardless, it need to be fixed so I guess I'll find out...

 

A piston in the caliper being pushed back allows less fluid so in theory the pedal would not be so low.

Then it is possible you have a bad connection allowing air to be drawn in when the brakes are applied .

Air can be drawn in and no loss of fluid.

Where did you get your Master Cylinder ??

 

I got it at rockauto. Something that had the "heart" next to it so I figured it was popular, haha.

I am still trying to figure out why this only happens after driving. This morning when I fired her up the pedal was perfect. I let it idle for a good five minutes, pedal stayed perfect. Then drive down the street and it goes much further down. This can't be the master.

The idea with the piston pushed back is that is has to travel further to make contact with the rotor. Pushing fluid back into the master ends up with fluid in the reservoir, it doesn't affect where the pedal stroke begins, if that makes sense.

 

Oh lord, crazy wheel bearing play in the front right wheel. I think this is the source of the issue. Must have happened while wheeling last time. I will get this corrected and report back.

 

Just updating this because it is finally fixed. Replacing that wheel bearing did it. Brakes are fantastic now