97 C150 have a "residual pressure valve" for the rear drums a

[Skip Fix]

JB5. I tired the larger piston calipers before and it was even worse. Went back to stock calipers.
My 1978 Trans Am MC have a RPV in the rear drum section.
RPV from Wilwood recommendations

Residual vs Proportioning Valves and What Your Brake System Needs

Today we take a look at the differences between proportioning valves (both fixed and adjustable models) and the residual pressure valves we offer in this buyer's guide.

www.speedwaymotors.com

 

[Schurkey]

You have the horrible leading-trailing shoe rear brakes. Crappy pedal is epidemic with them.

Verify that the rear brakes are properly adjusted. They adjust by using the park brake, and nobody uses the park brake. Assure that all the usual stuff is still properly functional--wheel cylinder not leaking fluid out, or air in. Shoes in reasonable condition, drums not scored, rear axle shafts 'n' bearings not wiped-out, etc.

Verify that the rotor/hubs up front have properly-adjusted wheel bearings, and the calipers aren't sticking on the guide pins, and the caliper pistons aren't partially-seized.

I'd tell you to assure that the pressure-relief valve in the master cylinder that controls pressure in the Quick Take-Up portion is not leaking or weak...but I have no idea how to do that. I guess you're just supposed to replace the master cylinder.

Bleed the ABS with a scan tool, then bleed the rest of the brake system. See attached.





The REAL fix for the craptastic rear brake problem is to scrap the rear axle. Move up to a C2500 6-lug that has the decent/wonderful 11.x Duo Servo brakes instead of the horse-spit 254mm (10") Leading-Trailing shoe drums. But then you need to deal with the 5-lug vs. 6-lug issues.

Replacing the leading-trailing shoe brakes on my '88 made MORE difference than upgrading the front rotors/calipers, and master/booster from JN3 to JN5.

 

[GrimsterGMC]

I f you go to the correct manual for your truck and read up on how the combination valve works I think that will answer your question, it is a commonly overlooked but important part of the braking system that does a lot of important things. It is too long for me to explain here but you shouldn't start changing braking components until you understand how this part works, especially the master cylinder as it is governed by line pressures from the master cylinder.

 

[Skip Fix]

I do know what brake systems do I have race cars we have done all kind of different design to get specific parameters. This truck I have had since new Chevy dealer as well as shops with TechII in the OBD port have bled correctly-it is NOT a bleed issue.Probably something related to the ABS as over the years I panic stop and engage the ABS and the pedal gets firmer! They are better short term after an adjustment. Everyone that has worked on these vehicles knows they have a crappy pedal feel from the factory.

Many on here and other truck forums as well as Stainless Steel Brakes recommend the MC swap as a fix as it is supposed to direct a little more pressure to rear then the factory MC. Just not much help on mine. That evidently is supposed to NOT trigger the self adjusters to work that more pressure is supposed to.

Since Wilwood manufactures brake systems and recommends a RPV with drums you would think they know what they are doing.

Rear brake lines to flex a metric?

 

[Schurkey]

it is NOT a bleed issue.Probably something related to the ABS as over the years I panic stop and engage the ABS and the pedal gets firmer!

If you're convinced that the problem is the ABS, why not get a properly-functioning ABS unit, instead of screwing with an improper master cylinder?

An ABS solenoid valve that should be normally-closed, but leaks into the accumulator would cause a low pedal.

They are better short term after an adjustment.

How often do you use the park brake? Be honest.

Everyone that has worked on these vehicles knows they have a crappy pedal feel from the factory.

Define "these vehicles".

If you mean "trucks with the crappy 254mm leading/trailing shoe drums" I might agree. If you mean "every GMT400 including the ones with 11.x or 13 inch rear drums" I'm going to disagree.

Many on here and other truck forums as well as Stainless Steel Brakes recommend the MC swap as a fix as it is supposed to direct a little more pressure to rear then the factory MC. Just not much help on mine. That evidently is supposed to NOT trigger the self adjusters to work that more pressure is supposed to.

If the problem is the rear brakes, why are you screwing-up the front brakes with a master cylinder that isn't intended for Quick Take-Up calipers?

"More pressure to the rear" is a false assumption, in that the pressure front and rear is equal until it's been modified by the ABS and/or the combination valve. If there's more pressure to the rear, there's more pressure to the front, and the vehicle stops faster. Different master cylinder doesn't change the brake balance.

Since Wilwood manufactures brake systems and recommends a RPV with drums you would think they know what they are doing.

They recommend stuff that makes them money.

 

[Erik the Awful]

Many on here and other truck forums as well as Stainless Steel Brakes recommend...

Be careful with internet recommendations. Over a lot of threads Schurkey has laid out pretty solid reasons why swapping a non-quick uptake master cylinder onto a quick-uptake system is bad. The internet "experts" aren't going to donate to your legal defense if opposing counsel finds out you're running a later master cylinder.

 

[Skip Fix]

Shurkey-"These vehicles" GMT 400 1/2 ton trucks mainly after 4 wheel ABS systems started possibly the ones with the shoes you describe. My 92 never had an issue.

Stainless Brakes tech line and catalog recommended the non Quick take up , but i agree a QTU is probably a better choice for "low drag" calipers-why GM switched in 81.

i will say the Mopar non quick take up MC on my 79 Camaro race car that used "low drag metric" calipers on the rear works fine with no pedal lag and i can watch the pressures on my gauges go up at the same time. front and rear. So yes a non QTU master can work fine with calipers that originally used a QTU MC

Erik-original owner since new and i drove the truck for years with the original MC-swap maybe a click better. I still personally think it is an ABS design issue also, as engage the ABS and suddenly pedal stays good and stops better for awhile. My bet using a T up front and either an adjustable in the rear or a combination valve for the rear drums would be a great trial!