Alright y’all, I’m at the end of my rope with this thing. It’s been a month of non-stop (no pun intended) brake issues.

Issue: when driving, I get no pressure on the first press of the brake pedal, it goes to the floor. I have to essentially ‘pump’ it two or three times to build pressure to stop the truck.

Simple fix! I thought.....I’ve now replaced the master cylinder (three times! ˟˟˟˟˟˟ Duralast ), new drum kit + wheel cylinders, new Load sensing proportion valve, new brake booster, and damn near 2 gallons of brake fluid flushing everything.

There’s no leaks, I’m not losing any fluid anywhere either.

Here’s the thing, every time I flush the system the brakes tighten up and feel great for about 5 miles. Then they go back to the same thing where I have to pump them to build pressure. There’s no air in the system when I re-flush once it starts losing pressure, so it’s not sucking in air anywhere I don’t think...?

please give me some ideas or this thing is going to the yard.

‘88 4Runner (Prior 3.0, now a 3.4)

 

Forgot to mention, I’m getting a pretty strong stream of fluid out of all four wheels, and the LSPV when I’m bleeding.

 

First, I'd only recommend the AISIN (OEM) master cylinder, but others may differ on this view.

I'm fishing here, but maybe the MS you used isn't the right one for your truck, and thus the linkage gap isn't right. On that note, have you confirmed you have the right amount of free-play between the pedal linkage and the MS so it's not pressing on it when the pedal is in the relaxed (up) position?

 

yep, correct mc, and I do agree with you about the Aisin’s however my county doesn’t allow junk yards, and my dealership quoted me $800 for a new one so....RIP.

correct free play in the pedal, and it’s not pressing when it’s in the up. I’m double checking my pad gap in the front right now as well.

 

https://www.ebay.com/itm/Brake-Maste...QAAOSwDpte2nLQ
$80.

 

Sounds like air....
I read your post, but it still sounds like air somewhere in the system?
Just sayin... you don't have a caliper or wheel cylinder upside down? They won't bled right that way... don't ask how i know it can happen...

 

Are your rear brake shoes adjusted properly?

 

consistently needing to pump the pedal twice in order to have brakes is not an indication of air in the lines; you’ve probably bled them fine. the most likely problem is the rear shoes are too far from the drums. the first pump moves them into contact with the drum, the second pump applies the brakes front and rear because the slack in the rears was taken up.

 

Update, y’all, turns out master cylinder #3 is bad too. As I’m releasing the pedal, I can hear air getting sucked in through the rear of the MC culminating in bubbles rising up from the bottom of the reservoir. Not sure why this has happened to the last two, but both have been properly bench bled until there’s not even a spec of air in the reservoir or in the silicone hoses I’m using to recycle the fluid back into the reservoir. Both held pressure great for about 3 miles then lost seal.

it’s not around the hose fittings either, dish soap/water sprayed those and both are tightly sealing.

returning this duralast MC tomorrow; broke down and shelled out for the Aisin, it gets here next week.

in terms of shoe fitment, I adjusted the shoes back out when I did the drum job yesterday to the point I can barely hear each drum scraping as I spin the wheel freely. The cylinders and calipers also have bleeder valves above the lines, so that’s not the issue either.

guess I’ll just wait and see if the new MC fixes it!

 

Did you find an option for the AISIN MC somewhere cheaper than the $600 from your dealer? I hope so.

 

Make sure you bleed the the mc before installing the brake tubes.

 

Start at square 1, bench bleed the master cylinder, loosen your parking brake cable, adjust your rear brakes until they drag, then loosen them just enough to spin the wheel freely. then adjust your parking brake then bleed the brakes, then get back to us with the results.