Got an interesting one. When driving, my brake pedal feels normal (resistance at about half travel), until I push harder than I usually do, then it feels like it's going over a hump and drops to the floor. Brake force is then very minor and just about no resistance until the very end. Pedal just keeps going to the floor with almost no braking.

With the engine off, I can pump the brake pedal several times and the resistance comes back. I can hold down the pedal and it does not slowly drop the the bottom. It almost seems like the brake booster is causing the pedal to drop all the way, but that makes no sense to me. The fluid level in the reservoir is fine. I just want to make sure the problem is the master cylinder before I spend the $75 on a new one.

 

If you're not losing fluid and having to refill it, the seals in the master may not be holding pressure. Rebuild or replace?

But are you sure you're bled out OK? Needs to be done in the right order including the LSPV. That's worth retrying if you are strapped for the expense of the MC.

 

I haven't bled in a while, so that's due. Still, this happened overnight so it's like something drastically changed.

 

I suspect your seals in the master cylinder have just worn out. Brake fluid is harsh on rubber seals. The seals will eventually get spongy. I have bought part store cylinders and they will pass a bench bleeding test with me holding my fingers over the ports. You really don't have the ability to hold back that much pressure with your fingers but once it is installed in the truck, the pressure it takes can over ride the seals can give symptoms of what you are experiencing.

 

Thanks Terry! I'm not aware of anything else that can fail (lines, slave cylinder etc.) without fluid disappearing. I just felt it was weird that the behavior changed based on whether the engine was running, but I'm probably overthinking it.

 

With the engine off, the master cylinder only gets the pressure of your foot, which is likely not enough to overcome the seals. With the engine running, the vacuum booster is multiplying the force from your foot 3-5x, so marginal seals will start to bypass fluid and cause the pedal to sink to the floor.

 

I'm working with the same problem. What year is yours?

 

I had the same issue. normal driving with normal braking, then occasional slight bump then almost to the floor with no pressure. had to pump the brakes to get any pressure. bled the whole system, still the same thing. ended up being my master cylinder seals. id change it as soon as possible if you have already bled everything. another thing to check prior to doing the mc. check your soft lines with the truck running and someone pushing the brakes for ballooning. my cousin had this issue on his dodge and nearly the same symptoms.

 

yotajakes, what year/engine is your truck. Reason I ask is that I am having trouble locating a new master cylinder for mine, 89, V6. What was sent to me doesn't match up even though according to the numbers it should. I think I have it figured out but want to see if anyone else ran into this.

 

89 pickup. 22re. Oreilley had a reman unit in stock. 55 bucks with warranty. Had a larger body but everything else was the same.New unit was around 85. Low range offroad might have what you are looking for. What didn't match up on yours? Bolt pattern? Or lines?

 

Thanks, I haven't tried Low Range. I have used them before for axle parts for my LC. It seems the 22r master cylinder didn't change much and is readily available. I think it has to do with production dates between 88 and 89 when Toyota was developing Gen2 4Runners. The one most sites list now are for Gen2. What doesn't match up are where brake tubes exit the MC.
Just looked at Low Range. Same one as I ordered first time although they put a disclaimer regarding having to adjust brake lines for some models.
I'll see what shows up this week and go from there.

 

I've ran into this on my truck multiple times. Some 88 parts. Some 89. It's hit or miss as to what is what. May be easier to make or bend your existing lines.

 

I thought about that but I've messed with brake lines before with bad results. Much easier to get the right part.