Anyone ever replace their brake master cylinder?

I know how to do it just wondering if anyone has had to replace theirs on a second gen. Pretty sure mine's toast. Last night driving home I came down an off ramp, hit the brakes, and the pedal went to the floor. Pumped the pedal once, normal. It was perfectly normal for the next few stops then once again nothing. I had to drive it today and the number of normal stops vs. no pedal pressure is now about 50/50. No leaks that I can see but I won't be pulling the wheels until tomorrow. Just borrowed my brother in law's Tacoma in case I don't find the problem tomorrow so I can get to work on monday without dragging one of my summer cars out. I did just put reman calipers on the front within the last 6 months but I don't see any leaks. And when I have brakes they feel perfectly normal. Just like when the master started to go on a few previous vehicles I've had. Must not be common though as I can't find a master in stock anywhere nearby. But of course rockauto has them.

 

make sure the rear drums are adjusted properly. If not essentially you have to pump fluid to them first to fill up the excessive gap and then you get braking pressure as a whole. Hence why one pump to the floor, second is firm. The shoes / pistons retract back too far.

 

Yeah that's not the issue. Brakes are properly adjusted (they actually adjust automatically when you use the emergency brake frequently) and when they work properly there is zero pumping required and the pedal is completely normal. Every time I've had a caliper leak, or a wheel cylinder, or a brake line, the brakes required pumping to work at all and of course there were fluid leaks. But the few times I've had to replace master cylinders they have failed pretty much the same way as this. Only leaks I've ever had from a bad master leaked into the master cylinder. But I had a couple not leak at all. Once on a past BMW and once on the 87 Toyota turbo pickup I had 20 years ago. Totally normal then all of a sudden the pedal sinks to the floor. It also sinks to the floor slowly when stopped on occasion too which is another sign.

 

yes they are supposed to adjust automatically but LOTS of times they get frozen and dont adjust. I would check that before digging into the master cylinder

 

Oh believe me I'll be pulling the drums and pads and having a good look before I buy anything. With my budget lately I can't afford to just throw parts at it in hopes of fixing it. But everything points to the master. If it were the rear brakes out of adjustment the pedal would be soft all the time. It's not. One time it's perfect, the next it goes to the floor and the truck just doesn't stop.

Edit a few hours later....

So it appears that the hard line from the spindle to the caliper was weeping slightly at the caliper. Cleaned it, mashed the brake pedal multiple times, and it would be wet around the threads. So I just replaced that section of line and re-bled the brakes. No more weeping. Hopefully that was it but I haven't driven it yet. Also working on re-assembling the differential in my BMW today. It's all together now, just need to double check the pinion preload and that car will be hopefully be whole again in a few hours.

 

Well a bit of an update. That line wasn't it. Replaced that Sunday and it made no difference. Been driving my brother in law's truck for the last few days. Pulled calipers/drums and looked at all the lines closely. No leaks anywhere. So I bought a new master cylinder, bench bled it, installed it, pushed about 2 liters of fluid through the lines with my Motive pressure bleeder.... Still have a horribly soft pedal that sinks to the floor. Now on to research if maybe I've got air backed up in the ABS pump. This blows. At least it's consistently sucky now instead of intermittent.

 

yeah it sucks that the non 09+ TRD OR 4x4's have the separate abs unit and vacuum assist master. The master on mine is the electric boosted and you can take a completely dry line and have it bled in about 30 seconds due to the electric motor pumping fluid.

Its very possible there is air in your abs unit.

 

I made a bunch of stops engaging the ABS in my ice rink of a driveway. Good news is the brakes work exactly like they did before 70% or so of the time. Firm pedal and it doesn't sink to the floor under pressure when stopped like it did before. But occasionally I still get a super-soft pedal. But it actually still stops OK when it does that now. Just a little disconcerting. It's always fine when I first start out from the house. Every time. Back out of the garage hit the brakes, it's fine. Between there and the end of the road if I hit them they'll be fine. When I get to the end of the road it could be fine or it could be soft. I was also thinking maybe one of the calipers was sticking (both replaced in the last 6 months with remans) causing the fluid to boil which causes a soft pedal. So I took the pads out and checked all the pistons, all seems good there. Re-lubed the guide pins, cleaned everything, and took it out again. Same deal. Quite frustrating. Not sure if I want to drive it and see if I can find something obviously wrong driving it more or just throw in the towel and bring it to a shop, but I'm absolutely flat broke. At least I got my BMW back together today so I have a backup and can give my brother in law his truck back. Of course that car has summer tires on it and it's 25 degrees out.

 

FINALLY FOUND THE PROBLEM!!! After bleeding the brakes again today the old-school way with the help of my wife I got no change in the way the pedal would sometimes go randomly soft. But it always seemed to do it after a corner or a big bump at low speeds. On the highway it wouldn't do it. Well I was searching around and found a post on a 4Runner forum where someone had the exact same problem and even his dealer couldn't figure it out. On his way home one of his front wheel bearings took a total crap and started howling like mad. Well I just put a cheap ebay wheel bearing on the LF of this truck 6 weeks ago. So sure enough I jack up that corner and it's got tons of slop in it. So whenever the wheel rocked it would push the caliper pistons in and the soft pedal was them going back to where they needed to be. So I tightened up the bearing and all is good. Of course I don't expect that bearing to last much longer...

 

that will definitely do it!

There is a member on here that sells already pressed together hub assemblies for a great price, check the 2nd gen buy sell trade section.

 

That's impressive that you found that.

What's the plan now, OE Toyota bearing replacement or try another aftermarket first?

 

Most aftermarket bearings are perfectly fine. I took a gamble and went with a $35 one last time I put a hub assembly together. Turns out it wasn't worth it.... So when this one starts making noise I'll probably go with an SKF. That's what I put on my last 2 trucks and I put plenty of miles on them with no issues. I've got another hub, the bolts, and an inner seal already just need to pick up an outer seal and another bearing. Then cut the bearing off the old hub. A project for another day. Need to tear down the diff in my BMW AGAIN and get that back together next. I hate that damn thing. Of course Ian already knows that....

 

October 2022 I replaced all of my brakes. Powerstop brand from RockAuto. Had to replace the front drivers side caliper, 2/4 pistons were seized and not working. Rear drum brakes were replaced, all new hardware, shoes, drums. Front was all new pads and rotors.
I now have a problem with a spongey pedal, and my ABS doesn't work either. Took it to Brakes Plus for a diagnosis, and they did a brake line flush. Worked good for almost two weeks, but anti lock still doesn't work, and now the brake pedal is getting spongey again. Although I am mechanical inclined to about 50% of the rest of you, I am at a serious loss here. I need help, please.