mushy brakes only in 4-Lo? (1 Viewer)

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I just got back from doing the first part of the Slick Rock trail as well as the Corral Hollow trail in the Northern Sierras near Lake Alpine. Here's some photos for your time:

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Here's the problem:

My brakes have always been what I've considered pretty typical for a 20 yr old heavy SUV on 33's - a little vague but they stop fine as long as you don't treat it like a sports car. PO replaced both rear calipers about 3 years ago. Never had any braking issues in my year of ownership.

However, on the trail this weekend while doing lots of slow, technical work in 4-Lo with the center diff locked, my brakes started to feel mushier than usual with more pedal travel.

On top of that, at one point I actually think I smelled brake fluid.

I never lost brake pressure or anything, but it got to the point that it was taking a lot of force to stop the thing on steep declines.

Once I got back to the highway and into high range gearing, my brakes went back to their old selves.


Here's my current thoughts:

Option 1 - my symptoms have something to do with my ABS system? In 4lo, ABS is disabled so maybe by doing that there is some air being released that isn't an issue when the ABS is enabled again?

Option 2 - My LSPV was being worked pretty hard going up and down rocks slowly in 4Lo, perhaps the valve being pushed up and down was opening up a leak that isn't detected when the valve is stationary on road conditions?


I plan to bleed the system this weekend, but I figured I'd see if any of you have experienced anything like this. I'm also wondering if I should have a shop pressure test the system before I start bleeding as there's no point bleeding air out if a leak is just going to let more in.

Any thoughts?
 
Change all of your brake fluid, a complete flush it probably just got warm and the old fluid did not like it, some people like a higher temp brake fluid I have never used it .
 
4-lo downhill in an automatic is always questionable to me. Especially with larger tires. I'm sure someone here will tell me this is a bad idea but I often descend in neutral. Can lock wheels no problem without the torque converter pushing it downhill. This is the biggest thing I miss about having a stick shift.

In any case I would bleed them if there's any question at all, just to be sure.
 
x2; run a gallon of new brake fluid through the system, with a pressure bleeder.
If the LSBPV is leaking, you'll know it. When it goes, it leaks like sieve.
 
SNIP
However, on the trail this weekend while doing lots of slow, technical work in 4-Lo with the center diff locked, my brakes started to feel mushier than usual with more pedal travel.

On top of that, at one point I actually think I smelled brake fluid.

I never lost brake pressure or anything, but it got to the point that it was taking a lot of force to stop the thing on steep declines.SNIP

Any thoughts?

If this problem was worst going downhill, it may be the linkages on the LSPV were rusted up or it was otherwise not fully functioning. It's designed to accommodate proper braking force distribution in spite of the arc of changes in suspension travel. If it's not doing it's job, then there could be lack of proper braking action.

Highly second the idea of a thorough brake flush if you don't know the history of the truck.
 
thanks for the reply, guys. I will start with flushing and bleeding the system this weekend.

It's definitely not worse going downhill. Just seemed worse the whole time I was on the trail in 4Lo.

The part that's confusing me the most is that I smelled brake fluid at one point on the trail. However, my reservoir does not seem to have dropped in level at all.

Only other thing I can think is the soft lines are old and brittle, and now that I'm really flexing them they are showing small cracks or something?
 
I doubt it. The brake system is an on demand pressurized hydraulic system. If the hoses (I assume that's what you mean when you say "soft lines") were leaking you notice a drop in fluid in the master cylinder reservoir. Maybe your cap was loose and some splashed out on the manifold when you were driving. Just a thought.

If you had a leak, you'd know it. It'll find the pavement.

Get a gallon (yes, you do need that much) of brake fluid and a pressure bleeder. I like speed bleeder screws too ;)
 
The key is that you wrote "slow" then there is a possibility that you are loosing vacuum on your booster, therefore affecting the brakes, the engine was not being revved and by pressing the pedal and letting it go and doing so again and againg...well yes...it will be harder to stop the tank
 
You can build a pressure bleeder yourself from parts found at home depot or buy a motive unit.

My brake fluid was ugly looking.
 
The key is that you wrote "slow" then there is a possibility that you are loosing vacuum on your booster, therefore affecting the brakes, the engine was not being revved and by pressing the pedal and letting it go and doing so again and againg...well yes...it will be harder to stop the tank

If he was in 4-Lo the engine would have been in higher rev's with slower forward movement.
 
I doubt it. The brake system is an on demand pressurized hydraulic system. If the hoses (I assume that's what you mean when you say "soft lines") were leaking you notice a drop in fluid in the master cylinder reservoir. Maybe your cap was loose and some splashed out on the manifold when you were driving. Just a thought.

If you had a leak, you'd know it. It'll find the pavement.

Get a gallon (yes, you do need that much) of brake fluid and a pressure bleeder. I like speed bleeder screws too ;)

Ok silly question - DOT3 right? Any issue with me not knowing what the current fluid in there is?

And if I don't want to use a pressure bleeder, can I just do the 2 man method and the second guy gets a leg workout?
 
4-lo downhill in an automatic is always questionable to me. Especially with larger tires. I'm sure someone here will tell me this is a bad idea but I often descend in neutral. Can lock wheels no problem without the torque converter pushing it downhill. This is the biggest thing I miss about having a stick shift.

In any case I would bleed them if there's any question at all, just to be sure.

The 80 has some of the best compression braking available in comparable rigs. You know that you can manually shift down to lower gears for additional slowing?
 
ABS should be neutralized on the trail it is dangerous!!!!

ABS is cancelled when activating the CDL. This can be done manually if your rig is modified for it or if stock, it happens automatically when shifted into Low Range
 
Ok silly question - DOT3 right? Any issue with me not knowing what the current fluid in there is?

And if I don't want to use a pressure bleeder, can I just do the 2 man method and the second guy gets a leg workout?
Yes DOT3.
Bite the bullet. You'll never get a land cruiser brake system bled that way. As it is, you'll have to bleed it two or three times, to get all of the air out of the ABS. Ask me how I know.
 
I plan to bleed the system this weekend, but I figured I'd see if any of you have experienced anything like this. I'm also wondering if I should have a shop pressure test the system before I start bleeding as there's no point bleeding air out if a leak is just going to let more in.

I would think a leak that is goint to let air in would show up as a fluid leak
x2; run a gallon of new brake fluid through the system, with a pressure bleeder.
If the LSBPV is leaking, you'll know it. When it goes, it leaks like sieve.

The LSPV in my 80 seized up internally and was non functional. It was also severely limiting the flow of brake fluid to the rear brakes. Definitely something that gets overlooked. It needs to be bled too, before bleeding rear calipers.
 
The part that's confusing me the most is that I smelled brake fluid at one point on the trail.

I had older GM cars that had master cylinder seals fail, brake fluid eventually made it's way through the booster into the cab at the fire wall. Not sure the sand is possible with an 80, but might be worth looking at.

With the engine off, do you have a firm pedal if you stand on the brakes? Does the pedal stay up, or sink to the floor if you keep pushing on the pedal? If it drops master cylinder might be done!
 
I had older GM cars that had master cylinder seals fail, brake fluid eventually made it's way through the booster into the cab at the fire wall. Not sure the sand is possible with an 80...SNIP

In a recent similar discussion and having no experience with master cylinder failure in an 80 (knock on wood) yet, it was pointed out the 80's booster/master cylinder is reversed versus in relation to the firewall, so failures won't leak into the booster as they often do on other vehicles.
 

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