Rear wheel panic stop lockup
 

Twice now I have had to hit the brakes hard once in the rain and once on dry a road. This truck is C10 with a chevy parts front disc conversion that I did not do. I pulled the rear drums off and discovered what I think is wrong. The front shoe is the long shoe and from what I gather that one should be on the back, not the front. Is that true? And could that be causing the rear to lock up during a hard stop?

Re: Rear wheel panic stop lockup
 

Yes, long show on the rear.

As for the rears locking up, the brake proportioning valve maybe adjusted wrong with too much bias on the rear.

If it's not an adjustable valve you can get one.

Re: Rear wheel panic stop lockup
 

Thanks for confirming that.
Since I didn't do this conversion I have no idea what parts they used. This is just one of the many things this guy did wrong on this truck. I have adjustable PVs on my Z and AMX. I am going to convert to 4 wheel discs so I will be getting all different stuff and probably add a adjustable PV.

Re: Rear wheel panic stop lockup
 

I have a truck that I did the conversion on and have the same problem. Actually, the truck had the same problem before disc conversion.

Adjustable valve is the sure way to fix it. Put my adjustable only in the rear line.

I have another truck with the Wilwood proportioning valve that has the adjustment built in. I think it only dials down the rear brakes, not adjust the proportion.

Re: Rear wheel panic stop lockup
 

Adjustable PVs work there is no doubt about that.
Along with the rear disc kit I am also installing a Corvette disc/disc MS and a disc/disc PV. With all that I might not need the adjustable PV, we'll see.

Re: Rear wheel panic stop lockup
 

The rear of a pickup is light with no load so the rears lock up. With a full load they tend not do so. That's by design on old trucks. Another thing is that when the rears lock you can still steer.

Unless someone is worried about brake fade in the rear, disks are a cosmetic upgrade.

Re: Rear wheel panic stop lockup
 

Yeah, I don't get excited about putting rear discs on. The proper way brakes are supposed to operate says disc brakes on the rear are not needed. It's not only to save money that automakers went to front disc but not rear. The front pads should need replacement at least 2 to 1 with rear shoes or disc pads. My experience has been closer to 5:1. Usually when I do rear brakes it's due to a leaking axle seal or wheel cylinder. Many prefer discs to drum because they see disc brakes as easier to work on/change pads. I get that, and they look cool if you can see them. I may do rear discs some day, just because. But this problem is all about proportioning. These trucks running empty need a greater bias to the front

* One reason I may do discs on my K2500 is if I can't find new drum brake parts in the future. Also, on a 4wd a consideration is not retaining debris after running through deep mud and water

Re: Rear wheel panic stop lockup
 

I pretty much agree with all that. But any degree of better more consistent stopping power can't be a bad thing. I probably would not do this conversion if the PO hadn't mess up the drum brake job.

Re: Rear wheel panic stop lockup
 

We're thinking the same. I wouldn't tell anyone it's not a good idea. I may end up doing my shortbed that way. Just saying I don't get worked up over needing them. My project started with a freshly done no part spared rolling chassis with 4/6 drop from an abandoned project being parted out. My brakes will be good for a while (once it gets driven). My '72 4wd, I think, needs the same thing as yours. I did new brakes/bearings/seals on it and have had two scares with brakes. It's off the road with another problem and I want to address the brakes once I get back on it. Not driving it until I do. Actually, mine has the opposite problem. Seems like I only have front or rears barely grab. Seems like a proportioning issue or other line pressure problem

Re: Rear wheel panic stop lockup
 

I hate brakes!!!! I own 4 cars and 3 of them have had brake problems in the last two months. The Z got a frozen rear caliper that morphed into a discovered bad racing CV axle. Finally got that fixed when my 2001 Ford (sorry to use the F word) Ranger ABS took a dive. You cannot buy an ABS pump for that anymore, not manufactured. But searching around YouTube I discover there is free software that will most likely fix it. New rear brakes, fluid flush and running that software fixed it. The C10 was not broke just did not feel good knowing I was in trouble if I had to panic stop it. Hopefully with this swap I can dial it in to be safe. I never haul anything in it (that is what to Ranger is for) so setting the correct proportioning should be easy.

Re: Rear wheel panic stop lockup
 

The rear lock when empty can be addressed with a manually adjustable proportioning valve on the rear line as suggested. Also you could get an active proportioning valve off a newer model truck. (1990's and later) They use a valve that has a link between the axle and the frame. The theory being as the truck is loaded the rear will set lower. The valve is calibrated to recognize this condition and allow full pressure to the rear brakes. And when the truck is unloaded the valve will limit pressure to the rear brakes.

Re: Rear wheel panic stop lockup
 

Yeah I just learned about the newer rear mounted valve. But after I took the drums off to look at the brakes and saw the PO had the shoes reversed I decided to go discs rather than rebuild the drum stuff.

Re: Rear wheel panic stop lockup
 

That is really interesting information! I'll be sure to keep it mind. I'm using the Wilwood valve in my braking system. Turns out that leaving it wide open for my rear brakes (in my application) works for best braking operation.

Re: Rear wheel panic stop lockup
 

This photo is of a Toyota LSPV (load sensing proportion valve). I've only seem reference or pictures of them on '80-'90s import trucks.
ABS brakes have negated the need for proportion valves, even on trucks.
The arm swings up/down with the axle and adjusts the valve. The arm is actually fairly flexible and they even call it a spring. The arm flexes to delay response to bumps in the road.

Re: Rear wheel panic stop lockup
 

I had issues with mine locking up and one of the things that I did that helped greatly was sanding the very edge of the brake shoe material on the top and bottom to get that sharp corner off. One of my old mechanic friends told me to try it and what a difference it made in my truck.

Re: Rear wheel panic stop lockup
 

On mine if the PO hadn't put the shoes on the wrong sides it might have helped too, LOL.