ShawnG
ShawnG PowerDork
6/15/19 6:11 p.m.

The wife unit drives a 1999 K2500 Suburban with the 7.4L 

The brakes on these trucks suck. 

There are two brake packages for the 2500 (3/4 ton), one uses 2.5" wide rear shoes and the other uses 3.5" rear shoes but the same drum. The usual trick that everyone tells you is to use the 3.5" shoes from the 10,000lb package truck even if you only have the 8,600lb capacity.

Our sub is already the 10,000lb rated HD model and already has the biggest stock brakes I could stuff in it.

So, off to the internets I go and find SSBC makes an upgrade kit for around $2,000. No thanks, I don't want stuff I can't buy at a local parts store.

Next is a kit to convert to the GMT800 3/4 ton spindles and brakes. Again around $2,000.

So I start digging and find that the 1999 Sierra 3500 DRW trucks used the same spindles as the 2500 but with thicker rotors and larger caliper pistons. 86mm diameter vs the 80mm 2500 calipers.

2500 rotors are 1.30" thick vs the 3500 rotor at 1.47" but more meat in the middle with less fin thickness. 20lbs vs 24lbs so more thermal mass.

Made the swap to the 3500 brakes today and it made a huge difference. Less pedal effort and faster stops. Should help with the rotor warping issues too.

Floating Doc
Floating Doc SuperDork
6/15/19 6:53 p.m.

I'm glad to see this. I wonder if the modification will work with my 88 C2500.

ShawnG
ShawnG PowerDork
6/15/19 7:45 p.m.

Had a quick look on Rockauto, the '88 C2500 calipers are the same as the 1999 K2500 calipers I took off so you should be able to make the swap.

Rotors might be a different story with the 2wd rotors but even if you can't get a thicker, heavier rotor, the larger pistons in the calipers should make a big difference.

Run_Away
Run_Away Dork
6/15/19 9:33 p.m.

Crud, was hoping for 6 lug/1500 upgrades.

SkinnyG
SkinnyG UltraDork
6/15/19 10:38 p.m.

I thought I had links to offer, but they are all 6-bolt.

D52 calipers?

 

ShawnG
ShawnG PowerDork
6/15/19 10:52 p.m.

In reply to SkinnyG :

Much bigger than those. 

Maybe the 1500's got D52 calipers, not sure.

 

SkinnyG
SkinnyG UltraDork
6/15/19 11:39 p.m.

99-02 Silverado Master has smaller pistons, and improves brake feel considerably.

(Googling this right now anyway, as I'll be running GMT400 spindles and brakes on my '61 Apache)

SkinnyG
SkinnyG UltraDork
6/15/19 11:40 p.m.
stylngle2003
stylngle2003 Reader
1/23/20 7:58 a.m.
SkinnyG said:

99-02 Silverado Master has smaller pistons, and improves brake feel considerably.

(Googling this right now anyway, as I'll be running GMT400 spindles and brakes on my '61 Apache)

Yes, did the "NBS" master cylinder swap on our '96 Tahoe 2wd and it made a huge difference in pedal feel.  Also did 2500 calipers at the same time (80mm, i believe).  You need one brake line fitting adapter for the NBS master to work with the older trucks.  The boosters and ABS systems can also contribute to crummy pedal feel.

gearheadE30
gearheadE30 HalfDork
3/17/20 4:58 p.m.

I'm not familiar with the brake system on the big blocks and heavier duty brake setups, but this definitely applies to the 1500s. For much of the '80s and '90s, GM had the brilliant idea to go with a low-drag caliper design in an effort to reduce fuel consumption. It wasn't very effective, and it had the knock on effect of giving GMT400s in particular the terrible brakes that they are known for.

The low-drag calipers use a different angle cut into the caliper seal groove. These seals are square cross section. When the brakes are appled, the angle allows the seal to flex outwards before sliding across the piston surface, and when the brake pressure is released, the seal wants to become square again and pulls the piston back in further. In theory, this keeps the pads off the rotors and reduces unwanted drag. It also means that the pads have to move significantly before they can engage the rotor when the brakes are applied, much like the brake pad knock back many of us have experienced on the track and/or with failing wheel bearings. On the GMT400s I've experienced, you can actually feel the similar effect by double-pumping the brake pedal rapidly during a stop. The second pump will give you a nice high, firm pedal...but only until the calipers retract again.

To allow the brakes to quickly cover this gap, GM came up with a step-bore Quick Take Up (QTU) master cylinder with a pressure-limited bypass valve. In any case, this master cylinder has a large diameter piston that causes the calipers to close quickly, and once brake line pressure rises, the bypass closes and allows the smaller piston to provide the actual mechanical advantage for braking force. All of this still takes pedal travel, though. This system works best when you slowly apply the pedal until pressure ramps up, and then add force to do your stopping.

Panic stopping, though, is a big problem with this setup. I've never understood exactly how GM was okay with it. When you are in a panic situation and suddenly stomp the pedal, the pressure spike in the master cylinder closes that bypass valve and immediately uses the smaller diameter piston....which takes much more travel to take up the gap in those low-drag calipers! This is part of why GMT400s do that scary "whoosh" thing with no actual stopping when you stomp on the brake pedal. If your system is in good shape, you will still have brakes but with a very low pedal. If there is any air, degraded rubber lines, etc, you probably won't have enough brake force to trigger ABS on dry pavement.

The NBS master cylinder upgrade is a good one because it is a non-stepped master cylinder and doesn't have all the QTU bits. The diameter conveniently splits the difference between the QTU piston and the actual brake force piston, so you get some dead travel but a firmer brake pedal overall.

Not all GMT400s have low-drag brakes. I believe the 3500s and 4500s had conventional brakes, and others may as well. I don't know much about the Hydroboost systems on these. Additionally, 9C1/Z56 police package GMT400s got a different brake system that uses the standard 1500 rotors, 80mm non-low-drag version of the 2500 calipers, and the larger 2500 drum brakes and 1 3/16" wheel cylinders. To go along with this, they got a special master cylinder that is identical in size to the 1.25" NBS master....but still has a QTU valve in it. That valve appears to do nothing in this application aside from allowing some initial pedal travel before firming up. Police departments must have had issues with their Tahoes that were supposed to replace the 9C1 Caprice sedans, and so GM had to do something for them. Interestingly, the Tahoe Limited got these good brakes as well.

I have a "normal" GMT400, K1500 Silverado with the crap QTU system. Even with all new parts aside from the booster, it is pretty wimpy. If I'm towing any significant weight, I double pump the pedal almost every time I stop just out of habit to make sure the pedal is there if I need it.

I also have a Tahoe Limited, with mostly old brake parts that still work fine. That brake system is actually really good. Stops well, good firm pedal, etc. It's what all GMT400s should have been. The only problem I have with it is that 20 minutes of enthusiastic driving on curvy roads will start giving me brake fade. The drums in the back are quite large, but the bias is all in the front because of stock brake dive, so they don't do much and instead dump all of the heat in the small front rotors. I don't really have brake dive anymore, so I need to figure out how to adjust the bias a bit and see if I can find some more aggressive rear shoes.

 

Run_Away
Run_Away Dork
3/17/20 5:30 p.m.

In reply to gearheadE30 :

Dude, awesome post.

I had always wondered how the NBS master cylinder in a gmt400 could improve brake feel while being a smaller diameter, it seemed like it should have achieved the opposite effect. Thanks!

ColoradoBob
ColoradoBob New Reader
5/28/20 9:25 p.m.

I had a similar brake issue with a 1989 Suburban 2500 with a big block (8 lugs wheels, 14 bolt 10.5 inch rear axle).  It just didn't want to stop.  It's a generation older and I don't what components may be common (if any)

I installed a vacuum pump on the booster and that helped, possibly because we live at an elevation of over 6000 feet, but the big help was replacing the rear drums with discs from TSM https://www.tsmmfg.net/ . They use GM rotors and calipers, so replacements should be easy to get.  It didn't stop as well as my 2008 Suburban 2500, which has large factory brakes with hydroboost, but it stopped much better than stock.

I never looked into changing the master cylinder, so I can't say if that would helped or not.

jfryjfry (Forum Supporter)
jfryjfry (Forum Supporter) Dork
6/5/20 9:35 a.m.

Any idea about the old gm caliper (from, say, a 74 blazer)  coming in a larger than 3" dia piston?

i don't know (but doubt) if this is the same style discussed here.

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