Brake Systems Experts Chime In, Odd Brake Issue (1 Viewer)

[PNWGERMAN]

OK so I recently put an EBay Brake booster on the LC hoping it would make my brakes feel a bit better. It did help, but I am having an odd issue now that I didnt have before replacing the booster. Of cores my dumb ass already threw the original one out so I cant swap it back over to see if it makes a difference.

Here is whats happening. When I start the LC up the brakes feel great, just as good as my 2017 Tacoma. I drive the truck about a mile, and as I drive I can feel the brake pedal progressively and gradually get stiffer, and the truck slowly starts the "auto brake". By this I mean the truck will start to slow down by its self if I lift my foot off the gas. It gets worse and worse until the truck will come to a slow and gradual stop with in a few feet if I lift off the gas, and if I'm accelerating I can feel the truck is slower and the engine is working hard to over come the brakes. I can fix the issue temporarily by shutting the LC off and then back on again, but with in another mile or so of driving the issue comes back again.

Its like pressure is building up in the booster some how over time, until the brakes are partially stuck on. I don't think its the a caliper sticking because the issue goes away when I turn the truck off and on again. The push rod is adjusted correctly, I bought the tool to do it right.

So my question is, is it possible that the brake booster has some sort of internal leak that causes a gradual pressure/vacuum build up and keeps the brakes partially applied until I turn the engine off and the vacuum sores shuts off?

Anyone have a sores for a decent brake booster that wont be $600+? Maybe more like $300?

 

[PNWGERMAN]

Went ahead and ordered an original parts Cardon rebuild off RockAuto, as I'm almost 100% its the booster, but I'm still interested in the answer.

 

[Mark W]

Went ahead and ordered an original parts Cardon rebuild off RockAuto, as I'm almost 100% its the booster, but I'm still interested in the answer.

You have the pushrod in the booster (to the MC) adjusted a little bit too long.

Mark...

 

[PNWGERMAN]

You have the pushrod in the booster (to the MC) adjusted a little bit too long.

Mark...

I could be wrong but I adjusted it with a specific measuring tool for the job today I and am fairly sure I did it right.
If that was the issue why would it get worse slowly and go away after shutting the car off? If the push rod was to long and was always pushing on the MC a bit, wouldn't the brakes drag constantly?
Not trying to be a smart ass, just trying to understand the problem.

 

[Mark W]

When the pushrod is just a tiny bit too ling, it does not release the MC completely and you retain a tiny bit of volume every time you actuate and release the pedal. This quickly begins to keep the pad/shoes in contact and then begins to build pressure. Basically it is a cumulative effect until it is enough to drag the brakes.

You may have paid attention to the adjustment, but this is the only thing I have encountered that will cause this and it always will cause this. I have seen guys online supposing that a failing brake line can create an "internal trap door" that allows fluid flow one way and not the other. Never seen it. Don't expect I ever will in reality. IF something like that is possible... the observed instances of mis-adjusted pushrods out weight any possible cases of clogged lines by several orders of magnitude.

Mark...

 

[jpoole]

It's really easy to determine if an over-long master cylinder pushrod is the problem. Grab a socket wrench and loosen the 4 nuts that hold the MC to the booster. Loosen all four nuts equally so that a small gap is introduced between the MC and the booster with a 3 or 4mm gap being all that's needed in a case where the brakes don't bind when cool. If the binding is immediately resolved then the pushrod is adjusted a bit too long.

Disclaimer: You can safely do a short test drive with the nuts slightly loose like this but don't drive or run the engine long with them loose as the nuts could vibrate off causing total loss of brakes! If the brakes were binding and bound before the MC bolts were loosened a quick back/forth stop/roll check in a parking lot is all that's needed to test. The rig should roll on it's own if you are on a slight slope, when the brakes are released.

If the above test determines that the pushrod is too long you can then remove the four nuts and pull the MC out a little further. Pull the MC off the ends of the studs but don't undo any of the lines so that you can avoid needing to re-bleed. Once there is more room between the MC and Booster push the brake pedal until the adjustable end of the pushrod is exposed, a short prop rod can hold the pedal in position. Grab the necessary tools, like an 8mm (give or take a mm) wrench and something to hold the rod from turning, long-tip vice grips are what I use, then shorten the pushrod a half turn (roughly), bolt it back up and try again.

If the pushrod is just a hair too long on the 80 it will drive fine, without binding, when cool but then develop substantial drag when warmed up. One Winter I did brake work on my '97 80 which included adjusting the pushrod. Brakes were fine for months then on a few hundred mile round-trip the weather warmed up. On the outbound leg of the trip it was cool and brakes were fine but on the return leg of the trip the temps were a lot warmer and the brakes developed a severe drag after an hour or so on the road. A quick, minor shortening of the brake pushrod totally resolved the issue.

failing brake line can create an "internal trap door" that allows fluid flow one way and not the other

I've seen this in the wild a few times. The master has enough strength to overcome the obstruction but when the MC is released the wheel cylinder/caliper doesn't have the strength to push the fluid back so the pressure in the cylinder/caliper remains elevated enough to cause brake drag. This is typically going to only happen on one wheel and depending on severity may cause noticeable pull/drag or may just cause extra heat on the impacted wheel.

Keeping an eye on wheel temps is a useful practice and is an early indicator for brake or bearing issues. A quick touch of all 4 wheels near the hub on a gas stop is up there with eyeing fluids and checking the back hatch for oil spots to stay ahead of issues while on the road. An infrared thermometer is the cleaner way to keep an eye on these temps. If you've got a single wheel that's hotter you'll want to keep an eye on it. If one side is hotter than the other the Sun is sometimes to blame so consider sun exposure when checking temps.

 

[Mark W]

I've seen this in the wild a few times. The master has enough strength to overcome the obstruction but when the MC is released the wheel cylinder/caliper doesn't have the strength to push the fluid back so the pressure in the cylinder/caliper remains elevated enough to cause brake drag. This is typically going to only happen on one wheel and depending on severity may cause noticeable pull/drag or may just cause extra heat on the impacted wheel.

Yep, I meant to mention that too... the fact that only one, or *Maybe* two wheels could be affected by the brake line failure scenario.

Mark...

 

[PNWGERMAN]

When the pushrod is just a tiny bit too ling, it does not release the MC completely and you retain a tiny bit of volume every time you actuate and release the pedal. This quickly begins to keep the pad/shoes in contact and then begins to build pressure. Basically it is a cumulative effect until it is enough to drag the brakes.

You may have paid attention to the adjustment, but this is the only thing I have encountered that will cause this and it always will cause this. I have seen guys online supposing that a failing brake line can create an "internal trap door" that allows fluid flow one way and not the other. Never seen it. Don't expect I ever will in reality. IF something like that is possible... the observed instances of mis-adjusted pushrods out weight any possible cases of clogged lines by several orders of magnitude.

Mark...

Thanks so much for the explanation. Make total sense. Ill re adjust things and see what happens.

 

[PNWGERMAN]

It's really easy to determine if an over-long master cylinder pushrod is the problem. Grab a socket wrench and loosen the 4 nuts that hold the MC to the booster. Loosen all four nuts equally so that a small gap is introduced between the MC and the booster with a 3 or 4mm gap being all that's needed in a case where the brakes don't bind when cool. If the binding is immediately resolved then the pushrod is adjusted a bit too long.

Disclaimer: You can safely do a short test drive with the nuts slightly loose like this but don't drive or run the engine long with them loose as the nuts could vibrate off causing total loss of brakes! If the brakes were binding and bound before the MC bolts were loosened a quick back/forth stop/roll check in a parking lot is all that's needed to test. The rig should roll on it's own if you are on a slight slope, when the brakes are released.

If the above test determines that the pushrod is too long you can then remove the four nuts and pull the MC out a little further. Pull the MC off the ends of the studs but don't undo any of the lines so that you can avoid needing to re-bleed. Once there is more room between the MC and Booster push the brake pedal until the adjustable end of the pushrod is exposed, a short prop rod can hold the pedal in position. Grab the necessary tools, like an 8mm (give or take a mm) wrench and something to hold the rod from turning, long-tip vice grips are what I use, then shorten the pushrod a half turn (roughly), bolt it back up and try again.

If the pushrod is just a hair too long on the 80 it will drive fine, without binding, when cool but then develop substantial drag when warmed up. One Winter I did brake work on my '97 80 which included adjusting the pushrod. Brakes were fine for months then on a few hundred mile round-trip the weather warmed up. On the outbound leg of the trip it was cool and brakes were fine but on the return leg of the trip the temps were a lot warmer and the brakes developed a severe drag after an hour or so on the road. A quick, minor shortening of the brake pushrod totally resolved the issue.



I've seen this in the wild a few times. The master has enough strength to overcome the obstruction but when the MC is released the wheel cylinder/caliper doesn't have the strength to push the fluid back so the pressure in the cylinder/caliper remains elevated enough to cause brake drag. This is typically going to only happen on one wheel and depending on severity may cause noticeable pull/drag or may just cause extra heat on the impacted wheel.

Keeping an eye on wheel temps is a useful practice and is an early indicator for brake or bearing issues. A quick touch of all 4 wheels near the hub on a gas stop is up there with eyeing fluids and checking the back hatch for oil spots to stay ahead of issues while on the road. An infrared thermometer is the cleaner way to keep an eye on these temps. If you've got a single wheel that's hotter you'll want to keep an eye on it. If one side is hotter than the other the Sun is sometimes to blame so consider sun exposure when checking temps.

Great explanation. Thanks so much. I didnt even think of the system warming up and causing things to expand, which is silly of me. I should know that. Ill give that test a try, and just gap all the nuts with some feeler gauges.

 

[PNWGERMAN]

So yesterday I put a 1/16" gap between the push rod and the MC. According to my wife who drove the LC into work today, the brakes were still acting up. I'm going to try and throw that new booster in tomorrow afternoon and see what happens.

 

[Mark W]

Keep us posted. Very curious to know what it turns out to be. Might be a new data point.


Mark...

 

[Saito]

Had an issue similar to yours when I first got my 80. The front brakes were engaging themselves and it was just the fronts. Replacing the master cylinder solved the issue for me. Went through the whole adjustment procedure and it ended up being the MC.

 

[PNWGERMAN]

Had an issue similar to yours when I first got my 80. The front brakes were engaging themselves and it was just the fronts. Replacing the master cylinder solved the issue for me. Went through the whole adjustment procedure and it ended up being the MC.

I have a brand new MC from Marlin Crawlers. Less than a year old. Hope that hasn't gone bad already.

 

[azusa]

Do you have a heat gun to verify is it the fronts or the backs or all four of them. Before you install the new Master cylinder Make sure you flush all the brake fluid out, you could have the wrong fluid in there ie power steering or engine oil. All the times that I've seen brakes locking up after warm up have been seals that are swollen in the master cylinder and proportioning valve. You can verify with a heat gun, also you can get a screwdriver and push the pads back just a little to see if they'll release until you push the brake pedal again.

 

[PNWGERMAN]

Do you have a heat gun to verify is it the fronts or the backs or all four of them. Before you install the new Master cylinder Make sure you flush all the brake fluid out, you could have the wrong fluid in there ie power steering or engine oil. All the times that I've seen brakes locking up after warm up have been seals that are swollen in the master cylinder and proportioning valve. You can verify with a heat gun, also you can get a screwdriver and push the pads back just a little to see if they'll release until you push the brake pedal again.

One interesting point is they don't stay locked up. If you turn the truck off, then restart, things go back to normal.
And I used brand new Lucas dot 3 brake fluid.

I am doubting my wife's report now. I've been driving the LC to work and back all week, and have had no issues. So I think the issue was the push rod adjustment after all.
How ever I am going to switch the booster over to the nicer rebuild anyways. The Ebay unit has the vacuum port in the wrong place, is built out of thinner metal, and showed up bouncing around in a damaged box. I just don't trust it.

 

[Mark W]

I am doubting my wife's report now. I've been driving the LC to work and back all week, and have had no issues. So I think the issue was the push rod adjustment after all.

I have learned never to completely discount the relay of symptoms or descriptions of behavior from anyone who has driven the rig. But I also NEVER give significant credence to their reports until I have verified what is going on myself. Either ignoring the reports, or giving them 100% credibility is almost always a mistake.

Mark...

 

[azusa]

One interesting point is they don't stay locked up. If you turn the truck off, then restart, things go back to normal.
And I used brand new Lucas dot 3 brake fluid.

I am doubting my wife's report now. I've been driving the LC to work and back all week, and have had no issues. So I think the issue was the push rod adjustment after all.
How ever I am going to switch the booster over to the nicer rebuild anyways. The Ebay unit has the vacuum port in the wrong place, is built out of thinner metal, and showed up bouncing around in a damaged box. I just don't trust

Yeah, If it had been filled with the wrong fluid, It would not have released right away.

 

[ITRJ]

have you checked to make sure that your calipers aren’t seized? I’ve seen the front seize up on quite a few Toyota trucks including my 80, 4Runner and FJ cruiser. It could cause poor brake performance and/or the brakes to drag. As for the brake booster adjustment, is this something you guys have found in the service manual? Typically brake boosters are set from the factory and shouldn’t need to be adjusted

 

[PNWGERMAN]

have you checked to make sure that your calipers aren’t seized? I’ve seen the front seize up on quite a few Toyota trucks including my 80, 4Runner and FJ cruiser. It could cause poor brake performance and/or the brakes to drag. As for the brake booster adjustment, is this something you guys have found in the service manual? Typically brake boosters are set from the factory and shouldn’t need to be adjusted

Calipers aren't seized. I got a cheap brake booster off ebay that showed up bouncing around in a broken box from from Asia. Even though it was suppose to be a direct fit, it was not, because the vacuum nipple was at about 5 o'clock instead of 10 o'clock like the factory boosters. It also weighs a lot less than the factory Toyota booster. The other difference I found today between the Toyota booster and the ebay unit, is that the Toyota unit has a bit of free play in the push rod. The eBay one does not. Not sure why but there it is.

 

[PNWGERMAN]

So I put the rebuilt Toyota brake booster on today, and it immediately made a difference. The brake pedal feel is much more progressive and smooth, where as the eBay unit always felt like the brakes came on way to fast and aggressive. Even with the correct adjustments to the push rod. The rebuilt unit also came with the push rod pre adjusted to the correct spec, the eBay one did not.
Lesson learned, don't buy cheap parts especially for incredibly important systems like the brake system. I did only because I got laid off for three weeks and was worried about spending money.

Don't buy a brake booster from this shop.

Pardon Our Interruption...

The Cordon rebuilt Toyota booster showed up very well packaged, with a warranty, and worked great right off the bat. And it was only about $200 shipped.