74 REPU Brakes Locking Up
 

Hey guys, hoping to get some suggestions as to what to look into here.

Basics;
1974 REPU
13B-REW Single turbo engine swap

I got the truck up and running a couple months ago after it had been sitting since the early 90's. The brakes were completely shot, I replaced the master cylinder with one from an RX-4 which is the same 7/8" Bore and looks identical, since they don't make the REPU master anymore. It bolted right in. I also replaced all soft rubber lines, front pads, rotors, rear drums, shoes, springs, and all four rear wheel cylinders. I bleed the brakes more times then I'd like to admit to get the pedal perfect. The only original parts of the braking system are the booster, all hard lines, front calipers, and the proportioning valve. It's worth mentioning the booster is in place but not connected with vacuum to the engine. It stops reasonably well for what it is.

With that out of the way, I've been slowly driving the REPU further and further after getting the new engine in and shaking it down and tuning. I even took a longish drive last week that was over 40 miles round trip which was by the far the furthest I have been, everything went great until yesterday. I was out street tuning some more and started to notice the truck just felt "sluggish". I noticed it would come to a stop on it's own without me pushing on the pedal. It got worse and worse to the point I had go wide open throttle (WOT) in boost to get the truck to move, and I pulled off the side of the road. At this point the engine starting to overheat, which has never happened before or even gotten close to it, and the brakes were completely seized up; pedal was rock hard and I couldn't move the e-brake an inch. I let it cool down for about 20min and then cracked the two lines on the master cylinder to bleed some pressure, pedal went soft and after pumping came back and I was able to make it home.

Today I checked the master and it looked overfilled, I removed some fluid and went for a drive again, brakes were great until about 15-20min into the drive I noticed they were starting to drag again and the pedal was getting progressively stiffer. This time I stayed close to home so I made back in the garage just before a repeat of last time- coolant temps were climbing again and the pedal went full stiff. I put the REPU on jack stands and confirmed all four wheels were locked up.

At this point I'm guessing it has to be master cylinder (brand new) or proportioning valve (original to the truck) related. What else could cause this? Has anything similar happened to anyone else? It happens progressively over time- brakes will work fine then slowly start to drag after being perfect, once they start to drag they get worse very quick to the point the system is locked with hydraulic pressure. It also just randomly started- I had taken the truck on longer drives before this without issue, nothing has changed in regards to the brake system.

TL DR:
74 REPU, most of the brake system replaced, worked fine for a while, now all of a sudden after 15-20min of driving the brakes will progressively start to drag and then lock up the point the truck can't move until waiting for 20min for pressure to bleed or the brakes to cool, confirmed all four wheels are locking up and it appears to be hydraulic related. Input needed.

I think, and I'm just guessing, your rear wheel cylinders are backing out with pressure which is causing the shoes to tighten up. There are little bars that lock the wheel cylinders from moving or adjusting themselves once you install the cylinder and get the shoes where they need to be. Check that and check to see that your brakes are releasing or backing off in the front when you let off the brake pedal (visually look for movement). That's where I would start anyway.

Maybe just check to make sure you don't have any serious heat sources close to brake lines. If your turbo or exhaust are close to a brake line it may be causing your brake fluid to boil.

There may be crud from somewhere dislodged and isn't allowing fluid to properly return from the calipers/rear cylinders. Who knows where that crud it, could be at the master, could be at the proportioning valve. But if you haven't taken the proportioning valve apart, I definitely recommend cleaning it out. I did mine a couple years ago and couldn't believe how nasty it was.

I would also check the rod length going into master cyl from brake pedal, make sure it has some slack when brake pedal is released.

Thanks for all the replies guys.

I suspected this initially but if a rear wheel cylinder was over-extending, wouldn't that only lock up that wheel? All 4 wheels are locked hard. I also did pull a drum and the cylinders appear OK.

Thanks for the reply, nothing close to a heat source, was careful when placing the turbo to avoid this.

That's a good idea. I did flush out the system extensively (almost two big bottles) the fluid was black at first and it's clean now and has stayed clean for a while but I never did clean the proportioning valve. Thanks for the idea!

I think this may be it; when replacing the master I adjusted the pushrod from the booster so it's basically touching the masters piston. I suspect as fluid heats up a little it wants to expand and push out and can not without any slack. I will be adjusting this tomorrow and then cleaning the proportioning valve.

Thanks for all the insights!