1996 K2500 8 Lug master cylinder HELP

[SixSpeedSS]

I need a master cylinder for my K2500 ECSB. I looked it up on RockAuto and they show 2 different Raybestos part numbers. One is 1.25" bore and the other is a stepped bore at 40mm & 28.6mm. So I go on Raybestos site and they lst the same two master cylinders for my truck. I called the GM dealer figuring I could give him my VIN and he could tell what belongs on it. He gave me a GM# 18029968, which crosses over to the 1.25" bore master cylinder on Raybestos' site (MC390257).

My question is this...Could the step bore MC (Raybestos #MC390259) be designed to correct the known issues with these trucks and the poor braking performance? The truck has new pads/calipers (done before I got it, but looked brand new), and I did new shoes/wheel cylinders/hardware in the rear. I also bought a Stainless line kit from Inline Tube and plan to replace all 3 hoses with stainless braided hose. Before I do the lines/hoses I want the truck to stop properly.

It seems that there isn't much pressure to the fronts. The rears seem to be doing all the work. It takes nothing to lock up the rear wheels and the truck does not stop all that well. The other day it started making a bunch of noise when coming to a complete stop, so I decided to pull the rear drums. They were completely full of brake dust. All the rivet holes werre filled and my guess is there was so much dust it was between the shoes/drum and making the noise. When I pulled the front wheels, there was almost no brake dust on/around the front pads. The fronts don't seem to be doing much.

I decided to try and bleed the system again. I am getting fluid at every wheel, the ABS unit, and the master cylinder. When I bleed the master cylinder, I get a strong pressure of fluid from the front 'smaller' reservoir of the MC (rear brakes) and just a dribble from the rear 'large' reservoir (front brakes). This is why I think it needs a MC. But since I am going to replace it, I want the best I can get for it. If the step bore MC is a 'fix' for these trucks, then I want to try it. I'm pretty confident that a new MC will make the truck stop better, but I want to make it stop the best it can. With the big rotors and 13" rear drums, this truck should stop pretty well.

My 454SS stops pretty well and it is completely stock braking system with Hawk front pads. The rears are still original. I understand it is a lot lighter truck, but it still aint no featherweight.

 

[Wagonbacker9]

Do you want my VIN to check at the dealer and see if the other PN is for the 6-lug? In exchange for my VIN just tell me what you find! lol

Before someone else comes in to say it: hydroboost.

personally I don't feel the swap is worth it, but you may want to look into it.

 

[94blu1500]

I'd be willing to bet that the other master cylinder is for a 2500 Light Duty. I'd probably change the soft lines to the stainless braided lines before i changed the master cylinder. With the front wheels off the ground can turn the wheel with the brake pedal pushed?

 

[SixSpeedSS]

I'd be willing to bet that the other master cylinder is for a 2500 Light Duty. I'd probably change the soft lines to the stainless braided lines before i changed the master cylinder. With the front wheels off the ground can turn the wheel with the brake pedal pushed?

The front wheels cannot be turned with the brake pedal pushed. If the truck is in gear and 4wd engaged, the rears stop before the fronts (at idle). I know it takes less pedal effort/fluid movement to stop a drum brake than a disc.

Why would I change the front hoses when there is barely any pressure from the master cylinder going to the fronts?

 

[SixSpeedSS]

Do you want my VIN to check at the dealer and see if the other PN is for the 6-lug? In exchange for my VIN just tell me what you find! lol

Before someone else comes in to say it: hydroboost.

personally I don't feel the swap is worth it, but you may want to look into it.

I have thought about hydroboost, but this is sort of a beater truck. I just want the stock setup to work.

Look on your service tag in the glovebox. Do you see JB5 or JB6? Mine has JB7, which GM# 18029968 crosses over to. GM# 18029967 crosses over to JB5 & JB6

 

[Wagonbacker9]

JB6 she has.

 

[SixSpeedSS]

Then you use the other 'step bore' MC. I got a hold of a set of brake pressure gauges from a friend. I hope to be able to check tomorrow after work. Then I'll decide whether I need a master or not.

 

[DGA1]

So what happened?

If you apply pressure to the brake pedal, while the engine is on and it keeps sinking till it gets to the floor, the MC is bad; it's leaking internally.

If you keep the truck off and pump up the pedal till it's really firm and not moving your booster is good.

Rubber line expansion is the other problem if they are old. $70-80 for all three stainless steel lines, or much cheaper for rubber ones.

I put the JB7 MC, one ton calipers, and stainless lines, on my K1500 and it stops noticeably better. My K1500 did come with a F44 package from the factory, so bigger drums and MC.

 

[SixSpeedSS]

So what happened?

If you apply pressure to the brake pedal, while the engine is on and it keeps sinking till it gets to the floor, the MC is bad; it's leaking internally.

Pedal never sinks

If you keep the truck off and pump up the pedal till it's really firm and not moving your booster is good.

pedal stays firm

Rubber line expansion is the other problem if they are old. $70-80 for all three stainless steel lines, or much cheaper for rubber ones.

planning to go with stainless braided lines. Was hoping to correct issue before changing hoses or lines. I have the complete Inline Tube stainless hard line kit to install.


I put a brake line pressure gauge on at each of the front calipers. Engine off, only get 300-400 psi. Engine running get 800-900psi. Only when holding pedal as hard as possible. When normal driving, who holds the pedal as hard as possible.

The thing that gets me is when bleeding the master cylinder, there is only a dribble of fluid when bleeding the front. The rear almost has a spray. I'm going to order a master cylinder and give it a try.

It appears that the proportioning is in the 4 wheel ABS unit. I hope this isn't causing me an issue since it is disconnected (electrically).

 

[DGA1]

I replaced my MC, because it was bad - a few months ago - and got quite a bit more than a dribble from the front port, when bench bleeding it. This is a JB7 MC as well.

Let us know how it goes. It's always interesting hearing about what people encounter with the brakes on these trucks. It really is amazing to me that they could be so bad.