brake valve help

[yocami]

I've got a 1987 3/4 ton 4wd regular cab longbed, with the C6P rpo code and a full floating axle, it has a load sensing proportioning valve and linkage but its corroded and Im unsure if it is still functional, I'll post some pictures when I can get to it. Im planning on replacing the master cylinder, both wheel cylinders and rear brake hose as well as the drum hardware and brake shoes, I'm thinking I could bypass the LSPV with a thumb screw proportioning valve and adjust it to about 50% if the LSPV isnt functional. I tried to bleed my rear brakes because the previous owner had a way over sized rubber gasket on the master cylinder and it tore apart and leaked the rear reservoir dry, I put the correct gasket on and refilled the reservoir but I could not get fluid out when bleeding, ive heard that the proportioning valve in the engine bay could prevent bleeding but my knowledge is limited with that aspect of the brake system, the master cylinder is getting replaced because I was unaware that fully pushing the brake pedal to the floor could damage a seal inside the cylinder when bleeding, the master cylinder was leaking towards the vacuum diafram housing. Any and all knowledge, info and advice is welcomed and appreciated!

 

[jcperformance]

Sounds like your combo valve might be tripped if no fluid reaches the rear—pretty common after running dry, you may need to re-center it (crack a front line and lightly press the pedal). Replacing the master is the right move if it leaked into the booster, just make sure to bench bleed it first. Bypassing the LSPV works but it’s kinda a workaround and can cause rear lockup, so I’d fix the master and valve first before deleting anything.

 

[Ricko1966]

They make a centering tool for the combination valve that will make your life easier. As for replacing the load sensing valve with an adjustable valve if yours is bad that's a great work around.

 

[Keith Seymore]

....

 

[Grit dog]

Yes that should work.
Not nitpicking but your 87 3/4 ton shouldn’t have a full floater if it’s original. 99% certain. But good to know when ordering parts.
I haven’t touched the LPSV on the 86 because the original brake lines are just too nice for me to hack up.
I also haven’t figured out where the reduced and full positions are on the valve definitively.
Currently not hooked up but all the linkage is there, just need a starting point to position the valve. It does move on its own though because most of the time it has light load brakes and you can tell when it rotates enough to “full” braking. That big truck stops with authority! Although it would probably lock the wheels up way too soon if it had skinny OE size tires.

 

[AuroraGirl]

Yes that should work.
Not nitpicking but your 87 3/4 ton shouldn’t have a full floater if it’s original. 99% certain. But good to know when ordering parts.
I haven’t touched the LPSV on the 86 because the original brake lines are just too nice for me to hack up.
I also haven’t figured out where the reduced and full positions are on the valve definitively.
Currently not hooked up but all the linkage is there, just need a starting point to position the valve. It does move on its own though because most of the time it has light load brakes and you can tell when it rotates enough to “full” braking. That big truck stops with authority! Although it would probably lock the wheels up way too soon if it had skinny OE size tires.

c6p would be the kind to get a full floater , over 8600 gvw

 

[AuroraGirl]

ubjec: HEIGHT SENSING BRAKE PROPORTIONING VALVE AND ADJUSTMENT Model and Year: 1984-85 C2, K2 AND 1986-89 R2, V2 TRUCKS​ TO: ALL CHEVROLET DEALERS​ THIS BULLETIN CANCELS AND SUPERSEDES DEALER SERVICE BULLETIN NO. 85-210, DATED JULY 1986. THE R2 AND V2 TRUCKS FOR 1986-89 HAVE BEEN ADDED, AS WELL AS SERVICE INFORMATION UPDATED. ALL COPIES OF 85-210 SHOULD BE DISCARDED. The brake balance on 1984-1985 C2 AND K2 Suburbans can be adjusted in order to enhance rear braking performance under lightly loaded vehicle conditions. 1986-89 R2 and V2 Suburbans were built with this new brake adjustment. In order to change brake balance, it will be necessary to adjust the height sensing brake proportioning valve. In addition, burnishing the brakes following the procedure below may be beneficial on some vehicles. The following service procedure should also be used on all 1984-89 suburbans if the height sensing linkage has been disconnected from the valve during routine service. Important: For vehicles exhibiting a hard brake pedal feel with a cold engine or upon quick brake reapply, reference Dealer Service Bulletin No. 88-311-5, dated July 1988.​ SERVICE PROCEDURE Note: Always make sure that the brakes are operational after performing brake work and before returning the vehicle to service.​ - Raise vehicle, axle must be allowed to hang free (no load condition). - Remove nut and disengage lever assembly from valve shaft (Figure 1). Rotate valve shaft to permit installation of plastic adjustment gage. Center "D" shaped hole of gage must seat over "D" shape of the valve shaft and gage tang must be positioned in valve mounting hole (Figure 2). Reinstall lever assembly to the valve shaft utilizing a "C" clamp or channel lock pliers to seat lever nylon bushing to serration on the valve shaft. Important: Do not drive the lever onto the shaft using the attaching nut. Doing so may result in improper valve setting.​ Reinstall valve shaft nut and torque to 8 - 11 N.m. (70 - 98 lbs. in.) Cut gage tang from body of the adjustment gage to allow the valve assembly to rotate freely (Figure 3). Lower vehicle and test brakes. SERVICE PARTS INFORMATION Model P/N Description Code Color ------- ------- ----------- ----- ----- C2,R20906 14061394 Gage A Green K2,V20906 15548904 Gage E Yellow​ BRAKE BURNISHING - On dry clean level roadway, accelerate to 55 mph., observe traffic following and when its clear - Apply brake at a deceleration rate of 8 ft/sec/sec until vehicle speed is 25 mph. A decelerometer must be used to achieve the proper deceleration rate. - Repeat this procedure at 1 mile intervals 15 times. Decelerometers are available from various sources, one of which is: Ammco Tool Company 2100 Commonwealth Avenue N. Chicago, IL 60064 WARRANTY INFORMATION For vehicles repaired under warranty use: Labor Operation Number: H0777 Use applicable Labor Time Guide for labor hours. You must be registered for see images attach You must be registered for see images attach You must be registered for see images attach

 

[Grit dog]

c6p would be the kind to get a full floater , over 8600 gvw

Really?

 

[Craig Nedrow]

my 86 C20 has a 14 simi floater, 9 1/2 inch. 3/4 ton 4X4 would have a 14ff, or as Aurura girl stated, a camper special would have a 14 ff, at least mine does, and the big brakes as well.

 

[Grit dog]

my 86 C20 has a 14 simi floater, 9 1/2 inch. 3/4 ton 4X4 would have a 14ff, or as Aurura girl stated, a camper special would have a 14 ff, at least mine does, and the big brakes as well.

c6p would be the kind to get a full floater , over 8600 gvw

Pretty confident 80s K20s C6P or not are semi floating axle. 70s were FF, not sure what year they changed or if they went back to FF in 87, but my 86 C6P which is by all accounts original has a SF rear axle and the landscape trucks I used to work on that were 80s K20s had SF as well. We had a 81/82 (bumper turn signals) and a mid 80s K20 and C20. All SF.
The 85/86 K20 C6P my buddy just picked up is also sf.
From some more reading the consensus is 81-up big block C20s truck or burb and c20 crew cabs got FF axles.

 

[AuroraGirl]

Pretty confident 80s K20s C6P or not are semi floating axle. 70s were FF, not sure what year they changed or if they went back to FF in 87, but my 86 C6P which is by all accounts original has a SF rear axle and the landscape trucks I used to work on that were 80s K20s had SF as well. We had a 81/82 (bumper turn signals) and a mid 80s K20 and C20. All SF.
The 85/86 K20 C6P my buddy just picked up is also sf.
From some more reading the consensus is 81-up big block C20s truck or burb and c20 crew cabs got FF axles.

it would make most sense to me to have the 14 bolt full float for the heavy 3/4 ton and 1 tons, but I guess it could be that way. they would have went to heavy duty 3/4 getting them in gmt400

 

[Grit dog]

it would make most sense to me to have the 14 bolt full float for the heavy 3/4 ton and 1 tons, but I guess it could be that way. they would have went to heavy duty 3/4 getting them in gmt400

I concur but the semi floater is a 7500lb axle which is plenty for any srw old square body. If you’re putting more than 5klbs payload in the back of any old c channel frame 3/4 ton regularly or long distance,you’re abusing the hell out of the rest of the truck. With any luck you’d break the axle before the frame which is an easier fix! Lol.
Overloading full floaters breaks frame before axles in these style of trucks. Seen it happen back in the day when these trucks were new and got worked like dogs. Of course the same people allowing or directing their employees to load em that heavy also trusted me at 16-17 yrs old to weld them back together….lol. So not so bright on either account.

 

[AuroraGirl]

I concur but the semi floater is a 7500lb axle which is plenty for any srw old square body. If you’re putting more than 5klbs payload in the back of any old c channel frame 3/4 ton regularly or long distance,you’re abusing the hell out of the rest of the truck. With any luck you’d break the axle before the frame which is an easier fix! Lol.
Overloading full floaters breaks frame before axles in these style of trucks. Seen it happen back in the day when these trucks were new and got worked like dogs. Of course the same people allowing or directing their employees to load em that heavy also trusted me at 16-17 yrs old to weld them back together….lol. So not so bright on either account.

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