Project Rear Disc Brakes

[JayTheCPA]

The spacers are not absolutely necessary. Toward their benefit, they do help improve handling by making the front and rear stances are equal.


In the GMT400, the front axle is 4" wider as compared to the OE rear width. So from a GMT400 perspective, by adding 2" a spacer to each of the rear wheels, it places the front and rear tires on the same place on the road. Not a big deal for concrete highways, but definitely a noticeable difference on blacktop with any depth of rut. Also helps from the perspective of widening the rear.


Seeing as the GMT800 axle is 2.5" wider than the GMT400, that should still leave the front axle 1.5" wider overall.

So, with the axle swap where the end result is the GMT800 axle in your Burb, this should leave the front still wider than the rear by 1.5". Your call as to whether adding spacers (in the rear) is necessary. Personally, I'd drive the GMT800 axle for a bit and then decide whether to add spacers (if they are made that small) to get an equal stance.


If the final decision is to keep the GMT400 axle and swap brake parts, if the budget allows, consider getting 2" spacers as they do help stabilize the ride and handling.

 

[Big T]

OK thanks for that explanation on the track widths and spacers, it's clearer now.

As for doing an axle swap vs installing the brake parts onto the GMT 400 axle, I did a quick measurement and the mounting plates are completely different. I think the simpler route is axle swap and moving the leaf spring perches and rubber jounce plates into the proper position is the path of least resistance.

We are in the middle of the GMT 800 front brake conversion. All the parts have been acquired for my '99 and the knuckles for Colby's '94. I will order the reams for the ball joints and tie rods this week. We'll do the reaming on all the knuckles first, then I'll do the install on mine. Colby still needs to acquire the parts, which total over $1,000 if your going with new and top quality parts. So he'll be a little bit behind me.

This 2500 HD showed up in the inventory of a Pick a Part on the 3rd. We had a rainy weekend so I took advantage of that. The front brakes were there, but from experience, with all the cores, fees and taxes, if you're buying them as cores, it's a wash. If you're buying them to use, that works. If they're still there during their 40% off sale at the end of the month, we'll grab them.

 

[Big T]

This morning I was driving behind a Chevy Express 2500 Van. Got lined up several times to eyeball the difference in track width between the front and back. It's about 3/4" on either side, which is exactly what the track width difference will be after doing the axle swap on my GMT 400. I have no worries with this axle swap.

SmithvilleD here is ahead of me on this GMT 800 axle swap project. Hopefully, he'll post up here on his results in the next month or so.

 

[Will L.]

If you don't get caught in road ruts now, you won't feel them with the new axle. Some parts of the country are horrible.

Worst car to follow them was the mid 80's suburu brat- yeah the one with the 2 plastic seat facing backwards in the bed. Only cool thing about that wanna be el-camino.

 

[Big T]

If you don't get caught in road ruts now, you won't feel them with the new axle. Some parts of the country are horrible.

Worst car to follow them was the mid 80's suburu brat- yeah the one with the 2 plastic seat facing backwards in the bed. Only cool thing about that wanna be el-camino.

Ever been to Spokane, aka SpoCompton? The freeways in town are concrete and they have major ruts from everyone running studded snow tire. You could literally just lock into them like tracks and let the car drive itself.

We don't have much of a rut problem in SoCal. We have water problems, or lack of water problems.

 

[Will L.]

Lack of water- never heard of it, oh wait...

Never been to Spokane. It's on the list.

 

[Big T]

First pic shows the brake mounting plate on the end of a GMT 400 axle.

Second pic shows the brake mounting plate on the GMT 800 axle.

Third pic shows the caliper mounting bracket on the outside of the mounting plate. It's about 3/4" thick.

 

[SmithvilleD]

Here's a pic of the newer AAM 10.5/14 bolt with disc brakes next to my truck's original Corporate 10.5/14 bolt with drums.

 

[Big T]

Here's a pic of the newer AAM 10.5/14 bolt with disc brakes next to my truck's original Corporate 10.5/14 bolt with drums.View attachment 47512

This is my inspiration. Just a bit ahead of me on this project. Looks awesome.

 

[Husker6.5]

So what do you do about proper brake bias in the rear when swapping from drum to disc? Still use the disc/drum proportioning valve on the original vehicle, or do you snag and plumb in as a replacement the disc/disc proportioning valve off of the donor vehicle? Or are they the same proportioning valve between the two vehicles? Thoughts? Ferminator?

 

[Husker6.5]

I sure would hate for somebody who did this conversion to be pulling a 12K trailer and have the rears lock up solid during a hard braking and jackknife their rig because the bias was wrong and there was too much rear-to-front.

 

[Big T]

I sure would hate for somebody who did this conversion to be pulling a 12K trailer and have the rears lock up solid during a hard braking and jackknife their rig because the bias was wrong and there was too much rear-to-front.

Have you seen the size of the GMT 800 brakes that are going the front of these vehicles? These rear disc brakes are proportionally smaller and it's visually obvious.

I'm not worried about towing as I have electric over hydraulic disc brakes on both axles of my trailer. I can adjust so the trailer does most of the stopping.

 

[THEFERMANATOR]

As to the proportioning valve, no, you cannot use the new style valve. You MIGHT be able to snag one out of a 3500 HD truck as I believe they came with the KELSEY HAYES 3 channel ABS like our's, but had 4 wheel disc brakes. Yes the pads are larger, but if you do the math, the new style GMT-800 calipers actually apply LESS total breaking pressure to the pads. The difference is those, it is applying the pressure to a larger pad surface, so it is almost a break even. As to the front to rear brake bias, this could prove to be an issue as disc brakes tend to need higher pressures than drums to work by design. The GMT-400 rear brakes is capped at either 600 or 800 PSI(depending on which proportioning valve you have. Stock 2500 SUBURBAN's was 600, but the TSB valve raised it to 800 with a more even pressure rise until 800 was reached. Pretty sure the trucks got the 800 PSI valve.), and most disc brake systems will reach pressures of around 1200 or so for the rears from the specs I have seen. It isn't much of an issue under light braking, but when you get down on them, the disc conversion is going to fall short once the proportioning valve caps the rears at the 600 or 800 PSI. Then you run into the issue of extra pedal travel as the new style trucks have a master cylinder with a good bit larger bore in it to move more fluid. And to me, they should have gone to an even bigger master cylinder bore(which GM did with the GMT-900 trucks) as I felt the pedal travel was to much on the newer GMT-800 trucks. These were just a few of the reasons I opted to stay with factory components. Yes I still have to deal with short pad life because the pads just are not that big, behind the hub rotors, but everything is balanced and works together with VERY little pedal movement when stopping. I do have stainless braided brake lines, but I don't normally move the pedal more than 1-1.5 inches to stop. I have a very firm pedal, quick response, and I can haul her to a stop in quick enough times for me.

 

[Husker6.5]

Bias isn't about the size, it's about how much hydraulic pressure/fluid volume is distributed Front-Rear and the delay sending that to the rear. When I upgraded my ex-Gf's '68 Cutlass convertible from straight hydraulic drums (no power assist) all around to power disc brakes front/drums rear, besides getting the correct vacuum booster, master cylinder, steel brake lines and changing the actuating rod from the brake pedal, I HAD to swap to the proportioning valve to an NOS GM A-body disc/drum proportioning valve to avoid locking up the rears even under light braking.

Ok so you have trailer brakes. Not all trailers do, though, like most car toters. Also, even without a trailer, in a panic stop situation, proper brake bias is critical to maintaining control of your vehicle. So again, especially with going GMT 800 in the rear only, and even front and rear, the bias is critical because it is figured based on not only the brake systems front and rear specifications, but also on the the vehicle's weight and weight bias front/rear.

GM's engineers (and all vehicle manufacturers) get paid damn good money and have available great resources just to make sure people don't die from faultily designed brake systems.

Even though the GM 400 and GM 800 proportioning valves might "look" the same externally (idk, I've never seen a GM 800 proportioning valve, so I don't know what it looks like), internally their valving and bore sizes could be different to account for the platform differences. Then there's the interaction with the ABS system to consider, too.

Just radically altering a factory brake system without checking everything out and ensuring the engineering changes are sound, well what do you have to lose? Besides possibly your life.

 

[Husker6.5]

THANK YOU, FERM! You must have posted your response while I was writing mine on my phone. Agree with you 100%!

 

[Big T]

Bias isn't about the size, it's about how much hydraulic pressure/fluid volume is distributed Front-Rear and the delay sending that to the rear. When I upgraded my ex-Gf's '68 Cutlass convertible from straight hydraulic drums all around to power disc brakes front/drums rear, besides getting the correct vacuum booster, master cylinder, steel brake lines and changing the actuating rod from the brake pedal, I HAD to swap to the proportioning valve to an NOS GM A-body disc/drum proportioning valve to avoid locking up the rears even under light braking.

Ok so you have trailer brakes. Not all trailers do, though, like most car toters. Also, even without a trailer, in a panic stop situation, proper brake bias is critical to maintaining control of your vehicle. So again, especially with going GMT 800 in the rear only, and even front and rear, the bias is critical because it is figured based on not only the brake systems front and rear specifications, but also on the the vehicle's weight and weight bias front/rear.

GM's engineers (and all vehicle manufacturers) get paid damn good money and have available great resources just to make sure people don't die from faultily designed brake systems.

Even though the GM 400 and GM 800 proportioning valves might "look" the same externally (idk, I've never seen a GM 800 proportioning valve, so I don't know what it looks like), internally their valving and bore sizes could be different to account for the platform differences. Then there's the interaction with the ABS system to consider, too.

Just radically altering a factory brake system without checking everything out and ensuring the engineering changes are sound, well what do you have to lose? Besides possibly your life.

We'll approach this incrementally with caution.

Yeah GM engineers get paid damn good money to make sure that people don't die from faulty brake systems, but IMO the brakes on the K2500 are faulty. You say they design them so you won't die. Then why did they need a Service Bulletin instructing the change out of the proportioning valves on the GMT 400 Suburbans? Why do the front brake pads on both my son's and my Suburban last just 15K miles? Sorry, but the braking system on these are a pretty piss poor design. Stopping at freeway speeds is like a prayer. Maybe the proportioning is a greater concern on the trucks, but I doubt it on the Suburbans with much greater weight in the rear.

We'll cautiously see what happens here. SmithvilleD is ahead of me on his axle swap. He's installing the GMT 800 Hydroboost, Master Cylinder and presumably the proportioning valve. We're going to do the GMT 800 front brake conversion and see how that goes. No plans for GMT 800 Hydroboost, Master Cylinder or proportioning valve on my Suburban, but will go there if necessary. I will be working on the front brake conversion in April, while watching SmithvilleD's progress on both front and rear.

My ABS has never worked and I've never been able to figure out the problem, so compatibility with the ABS is not a concern. Just another great GM brake design.

 

[Big T]

I haven't been able to find the actual specs on proportioning % and pressures for the GMT 800, but I did find this nice article:

http://www.superchevy.com/how-to/71398-install-brake-proportioning-valve/

I know from memory that the stock proportioning valve on our GMT 400 Suburbans delivered 600 psi to the rear brakes. I know the replacement proportioning valve recommended by the Service Bulletin increased that to 800 psi and I've done that mod. I found the following excerpt from the article to be of importance:

"Drum brakes benefit from something called self-energizing action where the leading shoe uses the rotating drum to help push the larger trailing shoe into the drum with additional force, requiring less hydraulic pressure to apply the brakes."

By implication, disc brakes need MORE pressure than drum brakes. Again, I'm not that worried about the disc brakes going on the rear. And if I still have problems, I'll install the adjustable proportioning valve in the article so that I can dial down the pressure.

 

[SmithvilleD]

You're correct to consider each of the points brought up above. That approach is why I decided to do the work to swap pedal rods & modify the firewall mounting plate so I could fit the hydroboost & mastercylinder combination that matches the two piston front/rear brakes.

The proportioning valve & how everything interacts when antilock is active is something I'll need to test to understand enough to say more about how well it all works together. I've been involved with a few bigger front brake upgrades on performance cars with antilock. Have to be aware that antilock, at the traction threshold, can conceal some brake bias issues because once it's active, the computer is compensating. But it's not compensating until enough wheel speed differential is seen to activate the system. So up until that point, any bias issue is still in play.

The front/rear bias on pickups is rather a compromise - because the rear wheel loading can vary so much. Before antilock, they had to be very conservative (& give up some braking capacity the rear system still had in it) to avoid rear brakes locking & rearend trying to come around. Antilock improved the situation with it's ability to compensate, once to the point of wheel slippage, for how much brake force the rear is allowed to see.

Practically speaking, still gotta test it at just below traction threshold, at threshold, & on slick roads to see how well this setup actually works. If the built in proportioning valve proves unworkable, I'll cross that bridge then.

 

[ak diesel driver]

http://www.wilwood.com/MasterCylinders/MasterCylinderValves.aspx

 

[Husker6.5]

Why do your pads on your Suburbans last only 15K? So many factors. Type/manufacturer of pads, driving style, mechanical issues (like caliper(s) hanging up/collapsed brake hose/piston(s) hanging up in the caliper bore), maintenance or lack thereof, who knows, but that's really poor service life.

My 98 K2500 Suburban has 38K on its set of front pads and they're 3/16" from the wear indicators. I'm running ceramic pads with drilled rotors up front with braided SS hoses and kevlar rear drum shoe linings. My driving is about 75-80% stop-and-go city and the rest highway. Every time I change pads, I also replace ALL wear items such as caliper slider bolts, boots, etc. and properly lubricate all moving/wear points with the correct lube. I have had no premature wear problems with the front pads on either the Burb, or my 94 C2500 C&C work truck.

The 94 work truck, with its service box full of tools and the ladders and siding brake on it, weighed in at 7900# "empty" and with a house full of replacement windows, entry doors and a sliding patio door (23 windows, three doors, three storm doors, a patio, four boxes of trim coil, lumber and a roll of insulation on one farm house job I did) the truck scaled at 10,280# loaded (why I plated it at 6 tons). I ran severe duty pads with drilled rotors and SS braided lines up front and kevlar linings on the rear. My driving was about 75/25 Hwy/City as I covered 2/3 of Nebraska, 1/3 of Iowa and parts of Missouri, Kansas and South Dakota. I still got 30K out of the front pads. I had to do the front brakes about twice a year since I was averaging 70-75K a year.

Oh, and I've never had any problems going "whoa" with either of them, even the 94 when it was loaded to the gills or pulling a rental trailer with surge brakes and a 700 series Bobcat on it, either.