Chasing mushy brakes caused by LSPV? (1 Viewer)

[SHIfTHEAD]

So I've searched and read and searched and read and can't find anything that really addresses this specific issue. The issue is mushy brakes, which is common, but there are a couple symptoms that seem a little different. Basically if I push hard the pedal will make its way to the floor. But that only happens if the truck is running. Shut off and the brakes pump up hard and will not sink to the floor. I had some pulsing when stopping and decided to address that as a possible part of the issue. New rotors and rebuilt the front calipers. Stops smoothly now, but still mushy. So here's what I'm wondering; could the Load Sensing Proportioning Valve be sending fluid back to the reservoir causing the pedal to sink to the floor once the brake booster is boosting my leg power? Is that even how the LSPV works? I can't find something definitive on how the LSPV works, but I *think* it is. Is this line of thinking accurate or have I got the technical workings of the LSPV all wrong?

BTW, the truck stops fine, and if I hit the brakes hard and fast it stops hard and fast. Just after a few seconds I'm out of pedal travel.

 

[-Spike-]

Your description suggests master cylinder failure.

 

[SHIfTHEAD]

Your description suggests master cylinder failure.

Well, that was my initial thought, but with the engine off I can push the pedal and it will hold. I guess the MC leaking past or the LSPV leaking past is just as likely with added boost pressure. Yeah, I suppose that makes more logical sense to start there than at the LSPV...so I'll pull that first.

 

[LandLocked93]

When did the mush present itself? Before or after work done to the brake system?
Have you taken it to place where a dude with a machine can perform a proper brake bleed job? Yeah it's $60 or so but it will give you a baseline with which to start.

 

[White Stripe]

How old is your master cylinder and lspv valve?

 

[SHIfTHEAD]

When did the mush present itself? Before or after work done to the brake system?
Have you taken it to place where a dude with a machine can perform a proper brake bleed job? Yeah it's $60 or so but it will give you a baseline with which to start.

I got the truck about 6 months ago and have only in the past couple months gone to work getting it ready for the road. The brakes have always been like this, and I guess the PO never noticed or cared. Judging by all the other preventative maintenance that has been ignored, probably the latter. As for the bleed, I have three typed of bleeders and used the pressure bleeder on the system when I reinstalled the rebuilt calipers, so I'm 99% sure there's no air in the system.

How old is your master cylinder and lspv valve?

The LSPV looks like it could be original but the MC doesn't look terribly old. I'm not sure if it's anodized and so doesn't corrode or what. Either way I've ordered another MC and should have it later today. If that doesn't solve it then I'll get another LSPV. And if that doesn't solve it I'll follow LandLocked93's advice!

 

[TerraNerva]

I was having same issues, and I ended up rebuilding front calipers. Pistons had seen better days. Pricey, but worth it IMO. My LPSV was shot, too, so I removed it and straightlined front and back. Has made a huge difference. It was a booger to bleed properly, took several attempts. System is loving that clean moisture free fluid.

 

[SHIfTHEAD]

I was having same issues, and I ended up rebuilding front calipers. Pistons had seen better days. Pricey, but worth it IMO. My LPSV was shot, too, so I removed it and straightlined front and back. Has made a huge difference. It was a booger to bleed properly, took several attempts. System is loving that clean moisture free fluid.

Yeah, I'm debating bypassing the LSPV as well. I understand the point of the valve, but think it's a bit of lawyeresque overkill in most cases. These trucks aren't the Unsafe At Any Speed Corvairs. And with a working ABS they seem like a backup to a backup.

I take it you don't miss the LPSV, right?

 

[White Stripe]

I don't know why you would bypass it. I would install a new one if it's the original from 25 years ago and adjust it until it stops right. It's a proportioning valve that adjusts with cargo load. It's useful when you constantly change cargo in the back. Rear brake pressure is critical to have right or else it can induce a sideways slide during emergency braking on low traction surfaces.

 

[TerraNerva]

I don't miss it, and I do like stopping. I've run without it loaded, unloaded, in the snow and offroad and never had any of the problems white stripe mentioned, but I certainly understand the concerns. I can say I've ony had to emergency brake one time with no issues, but of course your mileage may vary. The system works so much better now, but I attribute that to the new pistons and clean, new, moisture free fluid more than anything.

 

[White Stripe]

It sounds to me like your master cylinder pistons are leaking slightly. I would replace your master cylinder and booster if it's old at the same time. If the lspv is old I would replace that as well and adjust so it brakes so you have the front/rear braking bias characteristics you want.

 

[Emerald80]

I have mushy breaks like you are describing. So far I have replaced the Rotors and Pads on all 4 corners (needed to be done) Front Calipers (rebuilt, front right had a seized piston) and then the master cylinder (new OEM, thinking it was the cause of my mushy pedal). I Bled and bled and bled and couldn't get rid of the mushy brakes. When I was putting in my OME springs a couple weeks later I noticed the LSPV was leaking out the rubber boot. I deleted the LSPV and the brakes felt a little better but still acts like you are describing . At this point I'm thinking there is air trapped in the ABS or the ABS module is somehow internally leaking? I'm going to try a power bleeder on it next and if that doesn't work ill try bypassing the ABS to see what happens.

 

[jonheld]

No one mentioned replacing 30 year old flexible brake lines.

 

[Malleus]

FWIW, I have two 1995 80s. I have factory new brake response on one (I remember what my first, four year old 1994 felt like) and mediocre pedal response on the other (not as bad as the mushy brakes on my daughter's 1994, but that's another story).

I have rebuilt or replaced everything except the booster and ABS module on the weak '95, with no improvement at all. The other one has only a new LSPBV (I replaced it a couple of years ago, because it was leaking - the brake performance has been the same, before and after).

I do not think that removing/bypassing the LSBPV will solve your problem. Like you, I think the problem is air in the ABS module that won't bleed out. If you can find a dealer who still has a test module, get them to try bleeding the air for you; I haven't found any who still have them, even though it's a required tool for them. I think they all tossed them after the fully OBD-II compliant models came out and they could use Techstream to run everything.

Building one of the test modules has been on my list for years. It's not difficult, it's just a switching module, I just haven't made the time to do it.

 

[SHIfTHEAD]

I have mushy breaks like you are describing. So far I have replaced the Rotors and Pads on all 4 corners (needed to be done) Front Calipers (rebuilt, front right had a seized piston) and then the master cylinder (new OEM, thinking it was the cause of my mushy pedal). I Bled and bled and bled and couldn't get rid of the mushy brakes. When I was putting in my OME springs a couple weeks later I noticed the LSPV was leaking out the rubber boot. I deleted the LSPV and the brakes felt a little better but still acts like you are describing . At this point I'm thinking there is air trapped in the ABS or the ABS module is somehow internally leaking? I'm going to try a power bleeder on it next and if that doesn't work ill try bypassing the ABS to see what happens.

OK, that's very good to know. The LSPV isn't leaking on mine, so I will work from the position that it's either good or likely not the issue now. I'll replace the MC as I've got one coming and I don't know the age of the one on this truck.

FWIW, I have two 1995 80s. I have factory new brake response on one (I remember what my first, four year old 1994 felt like) and mediocre pedal response on the other (not as bad as the mushy brakes on my daughter's 1994, but that's another story).

I have rebuilt or replaced everything except the booster and ABS module on the weak '95, with no improvement at all. The other one has only a new LSPBV (I replaced it a couple of years ago, because it was leaking - the brake performance has been the same, before and after).

I do not think that removing/bypassing the LSBPV will solve your problem. Like you, I think the problem is air in the ABS module that won't bleed out. If you can find a dealer who still has a test module, get them to try bleeding the air for you; I haven't found any who still have them, even though it's a required tool for them. I think they all tossed them after the fully OBD-II compliant models came out and they could use Techstream to run everything.

Building one of the test modules has been on my list for years. It's not difficult, it's just a switching module, I just haven't made the time to do it.

So the subject of the ABS keeps coming up as I'm researching. In one post I found that a guy even had the unit rebuilt and it still sucked so he bypassed it entirely and the brakes were better than new. In another someone said their insurance WENT DOWN because the insurance co. looks at the very old ABS systems as a hindrance, which I found to be an interesting comment, to say the least. So I'm starting to think it is the ABS, or very likely, that is the issue here.

As for the test module you mention, do you have details on how to build one of these? Are there plans out on the 'net somewhere? I'd be game to build one.

 

[lumbee1]

When I first purchased my Cruiser, the braking performance was mediocre to scary. I installed new EBC Green Stuff pads and while they helped it wasn't a huge improvement. I noticed that going downhill with no load in the rear seemed to make braking worse. The LSPV is designed to function with a stock height Cruiser and is out of spec when a lift kit is installed. I replaced the LSPV with a union and removed the extra sense line. Braking performance was dramatically better to the point that I couldn't understand why anyone could complain about 80 series braking even with 35's and a lift.

Fast forward a few years and the Cruiser is pulling to one side with one of the front wheels hot and the other cool indicating a stuck piston. I replaced the front calipers but didn't clamp the soft lines which introduced air into the ABS pump. I tried my best to bleed the system but could not eliminate the long throw mushy pedal. To simplify the system, I removed the ABS pump and reused OEM lines to reconnect the four corners. Braking performance is consistent and powerful. I need to bleed the system again but I am very happy with stopping power now.

I really wanted to keep the ABS but I've read that first generation ABS was simply not that good and was more of a liability than a safety feature.

 

[thabruiser]

Unless I missed it in any of the previous posts, the one thing no one has talked about, and was the culprit of my "mushy" pedal. Check your rear caliper slide pins. Mine were frozen or very tight, and the boots protecting them were deteriorated. Right rear especially. Once freed, the pedal didn't have to compensate for extra rear caliper piston travel, pedal height was again normal, and all was right in the world again. Hope this helps.

 

[Malleus]

Unless I missed it in any of the previous posts, the one thing no one has talked about, and was the culprit of my "mushy" pedal. Check your rear caliper slide pins. Mine were frozen or very tight, and the boots protecting them were deteriorated. Right rear especially. Once freed, the pedal didn't have to compensate for extra rear caliper piston travel, pedal height was again normal, and all was right in the world again. Hope this helps.

^^^
Always the first thing to check/clean when replacing pads.

 

[Malleus]

OK, that's very good to know. The LSPV isn't leaking on mine, so I will work from the position that it's either good or likely not the issue now. I'll replace the MC as I've got one coming and I don't know the age of the one on this truck.


So the subject of the ABS keeps coming up as I'm researching. In one post I found that a guy even had the unit rebuilt and it still sucked so he bypassed it entirely and the brakes were better than new. In another someone said their insurance WENT DOWN because the insurance co. looks at the very old ABS systems as a hindrance, which I found to be an interesting comment, to say the least. So I'm starting to think it is the ABS, or very likely, that is the issue here.

As for the test module you mention, do you have details on how to build one of these? Are there plans out on the 'net somewhere? I'd be game to build one.

I don't have "plans" other than what's in my head, but the function is in the FSM. The testing module is just a collection of switches used to open and close the ports in the ABS unit. You could easily rig a board with momentary switches and connector housings for the wiring.

 

[thabruiser]

We happen to have the FSM ABS actuator tester at my dealership. I performed the functions as per the FSM (been a few years ago now). Now, yours just might have air trapped in it behind the valves. But when I active tested mine, I was disappointed to find nothing changed. It is a fun thing to play with (a bit dated...lol) but from my experience a good brake bleeding procedure will eradicate 99% of the air from these older style systems.