Ilovemountains

I’ve had an on-going brake issue in my 88 for a while now; I can mash the pedal to the floor and can not lock a wheel!! I’ve replaced the master cylinder, both calipers, both rotors "Brembo blanks", new Performance Friction pads, new Centric drums and shoes! I’ve bled, bled and re-bled the entire system by the book, LR, RR, RF, LF then LSV. I’m leaning towards my booster but it seems to be working, when I leave the engine off I can stand on the brake pedal, literately it stays rock solid... When I start the engine the brake pedal can be mashed to the carpet. Its not making any sense and I don’t know what else to replace, the booster, lines and rear wheel cyls are the only things other than the hard lines that have not been replaced, I’ve got no leaks or anything and like I said with the engine off I get a solid pedal. Any thoughts?? What exactly is going on in the brake booster on this thing when the engine is running vs off?

waskillywabbit

iTrader: (-1)

Replace the other 10%

tried4x2signN

Are the calipers stock? Your sig and what you've posted you changed are confusing me...

pruney81

maybe you got a bad master cylinder out of the box? Also, when you are bleeding it does it seem like there is pressure in the lines? Are you seeing fluid shoot out?

Ilovemountains

I forgot about all that in my signature… I replaced the master cyl. a few months ago and it didn’t make a difference. Last week I found my pass side caliper was stuck so I ordered a pair of matching remanufactured from Advance Auto… I had a new set of Brembo rotors on a shelf in the garage so I went ahead and swapped them since the pass side was totally blued from the caliper sticking. I also ordered new rear drums from Amazon.com and threw them on this weekend as well as the new rear shoes and PF Carbon Metallic pads. Basically over the last 4 or 5 months I’ve replaced everything but the booster, rear wheel cylinders and the lines. I have no clue why they brakes are still so weak unless it’s the booster. Can someone explain to me how they work in our Toyotas, I have a basic idea with the vacuum pressure and the diaphragm and how it aids the master cylinder, I just don’t understand how I can have a solid pedal but as soon as I start the engine and get vacuum it turns to mush. When I bleed the brakes Im getting good clean fluid with no air at all, the brakes are working and stop the truck quick but it doesnt have ABS, I mean at 30mph I should be albe to lock all 4 wheels in my mind, it doesnt even feel like its coming close and I can mash the pedal to the floor!

Ilovemountains

Nice! Im seriously tempted, all it would take would be new rubber lines, a booster and the LSPV... everything else is new!!! Its very frustrating. I bet its the booster... and has been all along!

xxxtreme22r

could be booster could also be low engine vacuum, double check to make sure that the check valve to the booster is not in backwards. also check the load sensing valve in the rear. You can also rent out a vacuum pump from autozone and hceck to see if the booster is holding vacuum. If not its an expensive fix. Junk yard part material.

Ilovemountains

Ill have to check the vacuum, nothing on the booster has been replaced since new. The LSPV was moved a little when I installed my lift springs. Im not sure if that would make a difference or not, what does that valve actually do?? I know the rod that bolts to the body opens the valve more when you have a load, does that allow more pressure to hit the rear brakes only? If so that wouldnt make sense becuase I cant lock any of them no matter how hard I try! Not that I want to lock them, my fear is that I couldnt do a panic stop if I had to empty, if I have my motorcycle on the back there would be no way to stop quickly if I had to, Ive not been driving it much because of this issue.... Ill double check the engine vacuum pressure this afternoon....

osamabinkeifen

mine act like this aswell! im thinking its my rear cylinders. if your booster goes bad it usually sucks brake fluid out of the master cylinder and drips on the floor mat inside the truck. if the pedal goes to the floor with engine on its mostlikely not your booster in my experiences. when messing with a vehicle w/ drum i usually replace the wheel cylinders first.

xxxtreme22r

the lspv to my understanding takes away pressure from the front and sends it to the rear when loaded down. Also keep in mind the more rear sag/lift without using a relocating kit for the lspv the more it will affect the lspv. As to the side effects of it affecting break-ability I have no idea. My guess is some loss as drums don't stop as well as disc but I doubt it would cause to to loose all power in the brakes. As for the booster it could be bad and not leak fluid. If there is any type of leak in the diaphragm it will basically cause a hidden vacuum leak in the motor. Doing the wheel cyls won't hurt and I think they are pretty cheap. I am almost ready to do all of mine. Hopefully I can just have the rotors and drums turned rather than buying them new.

As a side note, I never understood why Toyota didn't use a normal Proportioning valve, because there is nothing to shut off fluid flow to front/rear if you blow out a brake line.

4Crawler

iTrader: (3)

LSPV controls braking bias:
- http://www.4crawler.com/4x4/4R_suspe...ortioningValve

On the booster, engine off = no vacuum = no boost = rock solid brakes because you have no assist. Same as your power steering with the engine off vs. on. Might be you put on a smaller m/c and it can't push enough volume to activate the wheel cylinders all the way.

vital22re

iTrader: (2)

how well did you adjust the rear brakes? maybe they aren't fully adjusted.

MB4runner

Yeah this fixed a similar problem on my truck. Pedal is nice and firm now.

Ilovemountains

I adjusted the rear shoes till they rubbed the new drums then backed them down one or 2 clicks till they were not dragging but were right there. I have good pedal, it feels good when Im just driving around, its only when I really mash the brakes that it scares me, I can actually press the pedal to the floor with the engine running, if I turn it off I cant, they build solid pressure and the pedal is hard as a rock. It was doing this before I changed the master, that was my first guess at fixing the issue, I bought a rebuilt stock unit that was idential to the ASIN that came out.... The lift springs did come with a bracket for the LSPV that I installed when I put them on about 5 years ago, it compensated for the raised height of the rear end. All 4 brakes are working, just not like they should or used to. Im going to check the booster, Its not hissing and seems to be working properly. Its an odd issue I must say! As for buying rotors and drums, get your drums on Amazon, I got Centric C-Teks for $50.50 shipped!!!! They were $50 to $90 every place I checked each... As for the rotors, I lucked out and got mine a while ago on Ebay new in the boxes from a close out sale for $35 shipped for the pair .... The rear cylinders are not leaking and move the shoes properly so I cant imagine they are my issue, its got to be either the booster OR the lines are swelling and not holding the pressure to the calipers, I dont know but I will figure it out!

Thelast83inNJ

This is crazy but, could you clamp off each brake hose one at a time to see if you could isolate which wheel or end of the truck affects the sinking pedal? I've had brake hoses make vehicles pull, lock wheels,(won't release) and crappy pedal because it was flexing internally. Sorry.

hilux1026

Did you replace the flex lines going to the calipers??? could be that they are collapsing internally. What year is the truck? this could also explain the stuck brake caliper.

These lines do wear out over time and need replacing.

Blue_petersen77

If your push rod is not flush with the new master cylinder then that could cause your problem because you have to push the pedal farther before it pushes the piston in your mc. Make sure you adjust it correctly though or you could have problems. I adjusted mine out too far thinking that would give me an even tighter pedal ( it did) but then it didn't release the pressure all the way off the mc and caused the brakes to stay half locked when driving. I had to back it off so that the pushrod was just barely touching the mc piston when I pushed the mc all the way back to the booster.
Hope that helps.

LITLRED

I'm having a similar issue with my '86 runner. Brakes are hard as a rock with no vacuum but kind of mushy with the engine running. Hope you the cause. It will save me a lot of trouble.

Ilovemountains

I did check the MC rod, it was centered up good, I also set the pedal free play, which was only a little out due to the pedal being worn a little.. My truck is an 88 with 189,000 miles on the clock but has never been beat or rigged. I did not change the brake lines, which could be the issue though when I stand on the pedal, or have somone else stand on it with the engine off the do not bounce, move or change diameter, I checked them with a caliper. I didnt get time tonight before it got dark to check the vac pressure but will ASAP!!! I really wish I knew exactly what was going on in our brake boosters, perhaps there is a wearable part in there that is shot, not my diaphram??? Ive torn an Oldsmobile booster apart, there was nothing too it... Im hoping ours are more than a rubber seal / bellow...

Blue_petersen77

Does the fluid in the res go down over time? With all that you've done the only thing I can think of would be a small leak somewhere in the lines that you can't see or the flex lines swelling out... I'm curious to find out the cause of this particular problem.