Proportioning valve vs. load sensing valve

[13matsc]

Been doing some brake work on my truck recently. Had some issues last season and recently replaced all the hardware, shoes and cylinders on the rear wheels. Also replaced the master cylinder and the two lines to each wheel since the old lines where so stuck in the cylinders I had to grind them off.

Bled all the brakes and everything seemed fine. Jacked up the rear end and pushed the brake pedal - no rear brakes at all. I can spin the wheels by hand. Front brakes work good and parking brake works. I adjusted the shoes outwards as much as i could, but that ended up with one rear wheel overheating, and still no braking power. Now im not sure if I ever had rear brakes the 3 years i`ve had the truck….

My suspision is towards the proportioning valve in the front or the load sensing valve in the rear.
Grounded out the cable on the proportioning valve and that gave me the brake warning light, so at least the wiring from there is good. If the valve has been out of position i should have got the warning light - if the valve is in good condiotion that is. How normal is it for these to go bad?
The placement is horrible and virtually impossible to inspect without removing it.

Too many chances of snapping a line, so not keen on removing it if one cannot with a fair amount of certainty say its defect. How is the little connector on top removed?

Disconnected the lever at the LSV and tried moving the little switch all different positions, but no changes. I can hear and feel some sort of piston inside moving when i spin it around.

Put wd-40 on all the connections but i can just feel everyone of them not coming easily off. Already startet rounding off one nut at the LSV.
Would like to snap as few as possible

Any tips for a poor guy?

 

[Blue Ox]

Bleed them some more. Neither of those valves will make the brake pedal go to the floor.

The load sensing valve is an extra proportioning valve with a on/off switch. It's either proportioning, or it's not depending on ride height.

I hope you made a note of the position of the shaft in relation to the lever before you took it off and moved it.

Some more info about those valves here:

What is this? (LSPV Valve)

Any idea what this is on my 87 v20??

www.gmsquarebody.com

 

[13matsc]

Thanks. My bad, but the pedal is not going to the floor, but as far as it normally do.

I did take notice, as there were a little sticker on it, but it seems like this spins freely with maybe two positions.

Plan was maybe to delete the whole load sensing valve.

 

[Blue Ox]

Thanks. My bad, but the pedal is not going to the floor, but as far as it normally do.

I did take notice, as there were a little sticker on it, but it seems like this spins freely with maybe two positions.

Plan was maybe to delete the whole load sensing valve.

Your truck, your call, but without it my rear brakes would lock violently under anything more than gentile brake application. YMMV.

P.S. If you do remove it, I'd take it off your hands. They're scarcer than an honest politician.

 

[13matsc]

Thats the thing. Some people say this and other says the brakes got better after removing it. There even was a GM service bulletin to remove it.

Do you know the load capacity for when it was triggered?

 

[AuroraGirl]

Thats the thing. Some people say this and other says the brakes got better after removing it. There even was a GM service bulletin to remove it.

Do you know the load capacity for when it was triggered?

for the GM reasons its more or less "if the truck is outside of factory specifications, remove the thing." and that could mean clapped suspension, lift blocks, wider tires, etc.

 

[13matsc]

for the GM reasons its more or less "if the truck is outside of factory specifications, remove the thing." and that could mean clapped suspension, lift blocks, wider tires, etc.

Ah, OK!

Anyone with other thoughts on the problem? Could it be wrong cylinders in the rear?

 

[Old Guy Bill]

I know you’ve got old lines with seized connections but you’re gonna have to get some loose to find out where the brake fluid is.. or isn’t when the brakes are applied.
Use the proper line wrenches.
Replacement lines are readily available, or you can bend your own.
Always use anti-seize on the threads of line connections.

 

[mibars]

I recommend against removing it. It may change a hard braking into a potential head on collision before you have time to react.

From past experience with CRX I recall that removing proportioning valve was an easy "fix" to get the rear brakes to work more and to pass annual tech inspection with perfect results. Brakes were improved in everyday driving. It was all fun as long as the road surface was dry, on slippery surface it became plain dangerous with rear locking up easily.

 

[13matsc]

Anyone have any document or exploded view on How the load sensing valve works inside?

 

[SquareRoot]

Anyone have any document or exploded view on How the load sensing valve works inside?

It works like S**T! That's why you remove it and move on. You're in Norway huh? Whereabouts? I just got back from there. Awesome country. I saw everything from Oslo to Bergen, thru the rain of course.

 

[13matsc]

It works like S**T! That's why you remove it and move on. You're in Norway huh? Whereabouts? I just got back from there. Awesome country. I saw everything from Oslo to Bergen, thru the rain of course.

Seems like theres different opinions on that

Yep, rain and snow has gotten the better of us i southern Norway the last couple of years. Recommend you travel north the next time - nature is at another level.

Also spare parts for these kind of vehicles are available, but often from the states (Summit, LMC etc.) so if something breaks you can risk half the summer without the truck!

 

[Blue Ox]

Thats the thing. Some people say this and other says the brakes got better after removing it. There even was a GM service bulletin to remove it.

Do you know the load capacity for when it was triggered?

The installation was based on dimensions. That little plastic tab on the shaft was actually a sacrificial install tool. The tab was aligned with the arm, then broken off when things moved.

Basically there's a cam that the shaft moves that either allows the valve to move with brake application or not. The cam is spring driven so if the valve moved and the suspension traveled further it wouldn't break the follower off the valve. If you look at my pic in the thread I linked you can see what it all looks like.

 

[Jeff Lewis]

Replace proportioning valve. Mine was so rusted and hard to get to I hot knifed a hole in the plastic shroud to get to it from the top.

 

[13matsc]

Well, I started from scratch again and bled all the brakes. I managed to get the rear brakes working again, but still a huge amount of air in the system. I noticed a little «volcano» erupting in the front reservoir in the master cylinder when i pressed the brake pedal. I disconnected the lines from the master cylinder (which I bench bled and installed last year). Started bench bleading this in the car and the air does not stop bubbeling. I have pedaled for over an hour also tapping lightly with a rubber mallet in between and still bubbles. Can a defective seal or o-ring in the MC introduce air in the system? Only the front reservoir is bubbeling, not the rear.