Weird Brake Failure Issue (1 Viewer)

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You will also need a brake cylinder hone and hone the primary bore on the MC before you install the new kit to remove any pits or gouges. If you've never done this before, just buy the new unit unless you're Jonesing for a project. It's not hard, but you can put in the work and still have it fail.
That is sage advice right there, I tell you what.
 
I have an extremely odd brake issue. It started out a couple of times when I went to hit the brakes it went to floor with little resistance and zero stopping. I quickly pumped the pedal and had full strong braking. As you can imagine this was quite alarming. I checked everything I could and found some questionable vacuum lines. So I replaced everyone on the truck. A few days go by and it seems to have fixed it. Then it happened once again. I noticed that it was happening at slow speed low RPM thankfully. So I started digging again I initially thought maybe the booster went bad again. I had replaced it about two years ago. But it was passing all the normal testing. Then I was thinking an intermittent issue with the check valve. So I swapped in a new one and immediately was able to get the failure. Then this got me thinking it has to be something else. Especially since every booster failure I have every experienced resulted in a very hard pedal. Then the fact that the pedal is going through full travel to the floor with little resistance. Well yesterday I figured out how to replicate the issue every time. If I slowly apply the brakes it will go to the floor. If I apply the brakes at a moderate to fast speed it works as it should. Now this has got me thinking something is wrong with the master cylinder. Is there a way the seals could fail to cause this? It’s really about the only thing I can think of at this point. It kinda makes sense that the extra speed would build pressure fast enough to push against the seal and make it work. Any of y’all have any thoughts or experience with such a issue?

I've seen this on two FJ60s and both times it was the master cylinder. My thought is that the seals are worn and with slow braking the fluid passes the seals but with hard fast braking the fluid remains behind the seals. A new MC both times corrected the problem.
 
One thing to consider is loose rear axle bearings on full floaters. As you drive the axles will shift in and out causing the calipers to spread. The next brake application closes the calipers before having an effect. The second pump gives you regular brakes again.
 
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You will also need a brake cylinder hone and hone the primary bore on the MC before you install the new kit to remove any pits or gouges. If you've never done this before, just buy the new unit unless you're Jonesing for a project. It's not hard, but you can put in the work and still have it fail.
Definitely glad I went the R&R route then because I was slightly overwhelmed as it was (granted, I’m still somewhat new to this fun game, and it’s my DD), but to have it fail after it all would’ve driven me over the edge probably lol
 
Repair follow up:

Because nothing sux worse than an open ended repair threads. But it was 💯 the master cylinder. I now have the strongest brakes outta all seven 80s I have owned over the years. I thought my other 95 80 had some good brakes. With the new master it also feels more level under braking. Before I had a lot of nose dive going on. I wonder if my rears have been weak for a while. So the fronts were doing all the heavy lifting. Also would explain the very short life of my front pads.
 
Nice going dude, thx for the follow through. Looks like you got yours from Rockauto for $58.
 
Nice going dude, thx for the follow through. Looks like you got yours from Rockauto for $58.
Yep and it fit perfectly. It did not come with the threaded plugs for bench bleeding. I picked up a universal kit from NAPA for ten bucks. It had the plugs with nipples to run hose the hose into the reservoir. So it’s a one step bleed process.
 
Part number for gasket 47275-24010

If one deleted their ABS/LSPV could they get away with ordering a new MC for a vehicle W/O ABS like the one pictured? Would this be the recommended brand next to a Toyota branded unit? The rebuild kit cost as much as this MC

IMG_8171.jpeg
 

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