New front brakes won't lock up/big tires the problem?

[Supercharged111]

I have the bigger 8600# GVW calipers up front and they helped. The 14 bolt out back with bigger drums helped. The ABS delete and braided stainless lines helped with pedal feel. The Hawk pads helped. The GMT800 MC helped, but they still suck and won't lock up on dry pavement. I'm going to hydroboost it, all I'm missing is a high pressure line and GMT400 master meant for hydroboost. And the time to mess with it. IMO hydroboost is the only way on these trucks, I adjust my rear brakes every few months and purge brake fluid annually and it only ever makes it less bad. My suspicion is that the pedal over centers and stops pushing the rod into the master. That or the master flat bottoms out, both of which are massive engineering fails.

 

[bowtie-72]

I went through all this with my 1988 C1500. I've tried a bunch of different things to get decent stopping power. Here's what I tried and what worked/didn't work:

1. The original master was blown when I got the truck so I replaced it. There are two different size master cylinders for 88-91 trucks. You have a standard cab, so you're right you have the cheap tiny brakes and you need the smaller master cylinder. I got the bigger master cylinder and my brakes sucked.

2. I took one look at the tiny booster and decided that a lack of brake boost must be the problem. I had a 1993 C1500 years before and the brakes worked fine on that. It had a bigger booster, so I replaced the booster on my 1988 with the bigger booster. It helped, but the brakes still sucked.

3. I read on this forum about how people had swapped in the NBS master cylinder for improved braking. So, in went a brand new NBS master cylinder. I got a rock hard pedal, but the brakes sucked even more than before. Don't try this. The combination of the small D368 calipers with the NBS master will give dangerously little brake force at the front wheels. I could stand on the brakes hard enough to lock up the rear wheels and engage the RWAL on dry pavement but the front brakes just weren't doing much.

4. Swapped in better brake pads in the front. It had cheap autozone pads before and then I swapped in Centric Posi-quiet semi metallics. This brought my brakes back to about step 2.

5. After going this far and still having crummy brakes, I learned that the master cylinder changes I had made were making things worse. Going to the bigger master cylinder in step one reduced the pressure to brakes making them less effective. Swapping in the NBS master (it's even bigger) in step 3 made my brakes even less effective. The bigger master cylinder moves more fluid volume (this gives you the firmer pedal) but you have to input more force at the pedal to get the same force at the brakes as you had with the smaller master.

6. At this point, my brakes still sucked, but I knew a little more about how brake systems worked. I had two options in my mind to fix the brakes. Either install a smaller master cylinder, or bigger front calipers. I decided to install the 8600 GVW (D370) calipers and keep the NBS master. My thought was if the calipers didn't get my brakes to where I wanted them to be, I could still swap in a smaller master for more brake force. The 8600 GVW calipers with the NBS master finally gave me the brake force I was looking for. I can lock up the fronts at will and the RWAL doesn't engage before the fronts lock up anymore.

Disclaimer: I don't know if the 8600 GVW calipers will work on a 4WD with the tiny brakes. Hopefully someone else can chime in on that one.

Thanks for the heads-up on the master cylinder issue. I have a `91 reg. cab and I need to replace my master cylinder. I thought about upgrading to the larger cylinder, but I was afraid there may be an issue like you pointed out. (Point #1).

 

[alpinecrick]

The light duty 2500’s are 1500’s with the 14 bolt, 6 lug rear axle, the 2500 rear drum brakes, it’s own spring rate, and a larger driveshaft. The rest of the truck is a 1500–including the front brakes.

A larger booster may help a bit. Most trucks of that era originally came with 245 tires, going to 85 profile makes your tire about the same diameter as 265/75’s, and that is the most popular tire here in Colorado.

All my GMT 400’s run LT 265’s.
And yes, it takes a lot of effort to lock the brakes in these trucks regardless of tire size.

Making sure the rear brakes are in good shape can help in the stopping distance, too........

 

[gordonm1]

I removed my RWAL parts and it feels better but I need to test with a full load. Still seems like the brakes really only grab hard just as the pedal maxes out. Breaking begins pretty high on the pedal but this is lite breaking and not for an emergency stop.

I didn't notice any breaking problems the first ten years I owned the truck. Those years I was running the stock 225/75/16 tires. Now I run 245/85 I think and there is way more grip from the larger tires. The extra grip seems able to take more braking than the factory brakes were supplying.

 

[454cid]

Larger tires are like a cheater bar on a ratchet and your brakes are the nut.

 

[gordonm1]

I had a pro bleed the brakes to no affect and he insisted on another small master which was no help. He says it has to be the proportioning valve then. I was thinking the pedal goes to the floor because so much is going to the rear drums and the larger master would help but maybe I am stuck with trying bigger brakes or smaller tires or the proportioning valve.

 

[Schurkey]

I had a pro bleed the brakes to no affect and he insisted on another small master which was no help. He says it has to be the proportioning valve then. I was thinking the pedal goes to the floor because so much is going to the rear drums and the larger master would help but maybe I am stuck with trying bigger brakes or smaller tires or the proportioning valve.

There's a dozen things this could be.

What front calipers do you have? What size is the piston? Are they Low-Drag calipers? As far as I know, ALL 1500s got low-drag calipers. The early regular-cab 1500s got ****** small-piston low-drag front calipers clamping narrow rotors, the 1" master cylinder with a 36mm Quick Takeup piston in them, and the small booster. This brake system, JB3, is so bad that GM discontinued using it in the early 90's. My truck was equipped like this, and I ripped all that junk off in favor of the extended-cab front brakes.

Extended-cab 1500s got JB5 brakes, which included a bigger booster, bigger 1 1/8 bore master cylinder with 40mm Quick Takeup piston, low-drag calipers with bigger pistons, and wider front rotors. This system eventually became the standard even for the regular-cab pickups.

If you have low-drag calipers, DO NOT install the GMT800 master cylinder. It is completely wrong for use with low-drag calipers.

If you have the larger-piston calipers, the 1" master cylinder will lead to a low pedal.

The 1500 pickups (JB3 and JB5) got the terrible leading/trailing shoe rear brakes. That's a whole 'nother disaster. Rear brakes being out-of-adjustment is so common as to be epidemic. And then, yes, the pedal travel is excessive.