K10 14bolt swap now rear brakes lock up before the front

[Darty03]

A while back I swapped a 14bolt from I believe was an early 90s 3500 single rear wheel truck. I had to move the spring pads and shock mounts to fit my k10. It has the big 13" x 3.5" drum brakes. Every since then under hard braking the rear brakes locked up before the front on the road. Well, I used the truck for mostly on road so I lived with it. Now the truck has become a trail rig and this has become a much worse problem. I am having issues coming down steep rocky or muddy hills and the rear end wanting to come around on me. I just replaced the rear wheel cylinders and pads. The drums and hardware were good shape and dont have many miles on them. I though maybe the master cylinder being for the small 10bolt drum brakes might be causing an issues, but I see a ton of people running 14bolts with the stock 1/2ton master. Is this a proportioning valve, master sizing issue, or something else? The truck is a short bed and probably 200lbs lighter than stock in the rear. I built a custom flatbed for offroad that is mostly done and a big block swap, so lots of weight up front and not a lot in the rear.

 

[Strick]

Fot years I ran an 80 SB with a 14FF & 10 bolt 8 lug. I never had an issue with braking. Have you tried to ease up on the shoe adjustment to slow engagement?

HS

 

[Darty03]

I had them backed off more before I replaced the pads and all but they weren't abnormal, just what I would call looser than normal. I adjusted them tighter this time. No real change that I can tell but I can try backing them off even more than before.

 

[Ricko1966]

I had them backed off more before I replaced the pads and all but they weren't abnormal, just what I would call looser than normal. I adjusted them tighter this time. No real change that I can tell but I can try backing them off even more than before.

Are all the brake return springs in good shape and are the rear shoes in the correct position. Not always but most cases 1 shoe is longer than the other. Typically it is BOB big on back if you reverse them they are super grabby. And tge brake combination valve has a metering valve in it that delays engagement of the front brakes until the master has overcome the rear brake springs,that's why I asked about springs.

 

[Darty03]

Are all the brake return springs in good shape and are the rear shoes in the correct position. Not always but most cases 1 shoe is longer than the other. Typically it is BOB big on back if you reverse them they are super grabby. And tge brake combination valve has a metering valve in it that delays engagement of the front brakes until the master has overcome the rear brake springs,that's why I asked about springs.

All hardware was replaced a few years ago when the axle was swapped in and it only has 3k miles on it since then. The springs and hardware looked great shape when I replaced the pads recently. I did make sure to put the longer pad on the rear. I just double checked my pictures and it was that way before I did the pads recently too.

The prop or combination valve comment has me thinking it is working right? If I am on flat ground in the yard or gravel driveway and I stomp the brakes hard it locks up all 4 tires. If I am on the road and do the same like a panic stop the rears lock sooner than the front, and same with offroad on down hills in low traction situations. It seems like the rears just have so much more braking force? They dont lock under normal braking on road. The truck stops like I would expect it too except during hard braking on road.

 

[Ricko1966]

What kind of front pads are you using? Are front hoses and caliper slides in good condition. Maybe the rear isn't the problem.

 

[bucket]

What do you even have for front brakes? Still 1/2 ton 6-lug, with the smaller rotors? I think @Ricko1966 is onto something. The front brakes may just be inadequate, or a mechanical issue, or a truck weight bias issue. Or a combination of all.

Years ago, I put a ff14 bolt from a C30 under my K10 Suburban. The big and wide brakes. But I also put a K20 axle under the front, using the stock K10 calipers. The K20 rotors are bigger and do add some stopping power by themselves. The whole system works perfectly on road and off road. The only difference in our scenarios is my Suburban has more weight over the rear axle and I have larger diameter front rotors, compared to the K10 rotors that you might be running.

 

[Darty03]

What do you even have for front brakes? Still 1/2 ton 6-lug, with the smaller rotors? I think @Ricko1966 is onto something. The front brakes may just be inadequate, or a mechanical issue, or a truck weight bias issue. Or a combination of all.

Years ago, I put a ff14 bolt from a C30 under my K10 Suburban. The big and wide brakes. But I also put a K20 axle under the front, using the stock K10 calipers. The K20 rotors are bigger and do add some stopping power by themselves. The whole system works perfectly on road and off road. The only difference in our scenarios is my Suburban has more weight over the rear axle and I have larger diameter front rotors, compared to the K10 rotors that you might be running.

What kind of front pads are you using? Are front hoses and caliper slides in good condition. Maybe the rear isn't the problem.

I have no idea what the front pads are honestly but they are something from Napa or Oreillys. Everything is in good shape and moves like it should.

I do still have the 1/2 ton 6 lug hubs and brakes in the front. I plan to convert to 8 lug but a dana 60 is expensive now days. If the caliper brackets weren't so expensive I would have already converted the 10 bolt front to 8 lug. I'm saving up for a shop to work out of and then I plan to save for parts to build a ford dana 60. The big wide brakes is what this 14bolt has a well. I can try more aggressive pads on the front. I'm not sure if I will get enough heat in them to make them work like they should but it might be worth a try

 

[Ricko1966]

I'd try a performance oriented or heavy duty semi metallic pad do some browsing and comparing .Interesting comparisons here.

https://www.buybrakes.com/1985-chevrolet-k10/brake-pads

 

[Ricko1966]

Did you try changing to a good semi metallic front pad yet?

 

[Blue Ox]

Here we go again...

First off, you need to get the right brakes in the front.

Second, LSPV. Load Sensing Proportioning Valve.

It's the height sensing valve that modulates the rear brakes when the truck is unloaded. Plenty of people will swear it doesn't make a difference, but the two Squares I've owned with the 13" drums would lock violently without that valve. YMMV.

BTW, it's unobtainium. You may find one, and thr linkage, in a junkyard somewhere. Otherwise, add a manually adjustable proportioning valve in the rear line to tone those down.

 

[Darty03]

Here we go again...

First off, you need to get the right brakes in the front.

Second, LSPV. Load Sensing Proportioning Valve.

It's the height sensing valve that modulates the rear brakes when the truck is unloaded. Plenty of people will swear it doesn't make a difference, but the two Squares I've owned with the 13" drums would lock violently without that valve. YMMV.

BTW, it's unobtainium. You may find one, and thr linkage, in a junkyard somewhere. Otherwise, add a manually adjustable proportioning valve in the rear line to tone those down.

I was not aware the GMT400s ever used a load sensing valve (this axle is from that generation truck). That would make sense with the issues I'm having. I have done a ton of searching on here and other forums and havent seen anything about that. A manual proportioning valve was something I considered. I didnt want to double stack it on top of the factory proportioning valve and possible cause issues with it though.

Did you try changing to a good semi metallic front pad yet?

I have not yet, sorry. I've been busy with work and putting my free wrenching time into helping my brother with his XJ. I will update this thread when I do but unfortunately I'm short on time and its not going to be quick. I was looking at possibly getting the Wagner SX52 semi metallic pads. They are considered a "heavy duty" pad.

 

[Ricko1966]

I was not aware the GMT400s ever used a load sensing valve (this axle is from that generation truck). That would make sense with the issues I'm having. I have done a ton of searching on here and other forums and havent seen anything about that. A manual proportioning valve was something I considered. I didnt want to double stack it on top of the factory proportioning valve and possible cause issues with it though.




I have not yet, sorry. I've been busy with work and putting my free wrenching time into helping my brother with his XJ. I will update this thread when I do but unfortunately I'm short on time and its not going to be quick. I was looking at possibly getting the Wagner SX52 semi metallic pads. They are considered a "heavy duty" pad.

Did you read through that link sent you on post 9??

 

[Blue Ox]

I was not aware the GMT400s ever used a load sensing valve (this axle is from that generation truck). That would make sense with the issues I'm having. I have done a ton of searching on here and other forums and havent seen anything about that. A manual proportioning valve was something I considered. I didnt want to double stack it on top of the factory proportioning valve and possible cause issues with it though.

GMT400s wouldn't have used it because they had RWABS or later full ABS.

The squarebody trucks used it, and it was for all intents and purposes "double-stacked."

 

[bucket]

Here we go again...

First off, you need to get the right brakes in the front.

Second, LSPV. Load Sensing Proportioning Valve.

It's the height sensing valve that modulates the rear brakes when the truck is unloaded. Plenty of people will swear it doesn't make a difference, but the two Squares I've owned with the 13" drums would lock violently without that valve. YMMV.

BTW, it's unobtainium. You may find one, and thr linkage, in a junkyard somewhere. Otherwise, add a manually adjustable proportioning valve in the rear line to tone those down.

When I put the big C30 brakes under my Suburban, I considered using the load sensing valve that came with the axle. But I eventually chose not to. I never had any issues with rear lockup, even when braking down steep, muddy or gravel roads.

Oddly enough, my old '86 C20 Burb with 13" brakes, the load valve evidently cut too much line pressure to the rear. The front brakes had a tendency to lock up in a panic stop on pavement.