4 wheel disc brake lock-up

Searching back through all the post's I could find on rear disk conversions, I was unable to find a conclusion to "Don't Panic's" problem with his rear brakes. I have a similar problem that I am unable to solve.

I switched my 85 drum step up, over to an 88 rear axle/brakes. Added a SVO M/C, Gutted stock prop valve, placed adjustable prop valve in the rear brake line. I have the adjustable valve opened all the way, and can barely brake the rear wheels. The pedal is hard with no fade or mush at all.

I can stand on the brakes from any speed and none of my brakes front or back will lock-up, but the pedal goes down to 1/2 and stays there.

Any new ideas to an old problem?

I'll offer this observation. You went from a master cylinder with a 21mm piston (~7/8 dia) to 1 1/8 inch diameter piston. This increased your piston area by 36% and thus reduced the line pressure by the same amount, assuming no other changes in the pedal geometry. While a third more leg force doesn't seem like it would account for your observation, it's something to thing about. You may just have to "stand" on the brakes to get them to lock up.
I'm surprised that you get that movement in the pedal, the larger piston will displace more fluid and I would think would result in less pedal movement.
The following is the recomendation I have read on several web sites, the SVO MC (1 1/8) should only be used with the SVO (73mm diameter vs 60mm diameter pistons) front calipers and rear discs. As I recall the SVO rears are 54mm pistons and the '87/'88 TC are 45mm. So there's another small difference.
I don't think the SVO front calipers can be used with the '85 spindles, but can be used with the '87/'88 spindles. Just in case you decide to go with the later TC spindles, the strut mounting ears are 3/4" thick on the '87/'88 and 1" thick on the '85/'86. You'll have to use washers or make a spacer to use your existing strut.

I'd like to hear more from you as to how you hooked up the adjustable proportioning valve, whose you used and so on. I have the '87/'88 spindles, calipers and the rotors from an '85 Mustang on mine. I have the SVO calipers and MC to install as soon as I can get the '88 rear installed. So I'm real interested in how you set your's up.

Unfortunately, I didn't read enough posts, before buying the SVO MC and finding out that it could be too much, and it very well might be.

I had the original MC in with all the present mods and it acted the same way. Could be the same problem, with two different reasons? Or something else and is slipping past me? I don't know.

I used speed bleeders on the rear calipers and the brakes bled good, so I don't expect any restrictions in the line.

I used a Summit SUM-G3905 Prop Valve (though it says Ford motor sports on the valve itself) between the gutted original valve and SVO MC, using a 12" long 1/4" MC line from MC to the new Prop valve and a 12" long 3/16" line from the new valve to the old one (all from NAPA).

Definitely no expert on any of this. I try to read up on things and make the best decision possible. May have to try a different MC before I'm done

Oops, I think you've got the prop. valve in the wrong location, even though you gutted the OEM valve. The prop. valve should be after the OEM valve, between it and the rear brakes. Not sure how that might affect your setup.
I don't have a good answer on the MC. I've been told in other threads that cars with drum brakes have a residual pressure valve in them to prevent the wheel cylinders from fulling collapsing when the brake pedal is released. There's not as much piston movement required when the brakes are reapplied. But I can't find any confirmation of this in Ford's shop manuals. The net result is that drum brake master cylinders are not recommended for use with caliper brakes. Trying to find a master cylinder that's a straight bolt up for use with the 8.8 rear calipers seems to be hard to do.
Except for the SVO MC but then the front calipers need to be changed.

You could be right about the prop valve placement. I found the write up on CoolCats to be somewhat confusing in a few places, mostly due to it's being written for other cars in general and not specifically a turbocoupe, let alone an 85.

Was wanting to do the 5-lug swap next year, so not thrilled about switching spindles and calipers right now, but maybe I'll move the valve and see what happens.

Thanks

It doesn't matter if the car is a TC or not...the writeup is geared for drum-to-disc conversions. The larger Crown Vic/SVO master cylinder is mainly for a larger fluid capacity, since with rear discs you will be moving more fluid. Now your stock (hard) brake lines are smaller than a 1987-88 TC so that should help a little with keeping brake pressure up. But that's offset by the larger bore in the Crown Vic/SVO m/c and is something that simply must be dealt with...it's not really optimal for a drum-to-disc conversion but the larger fluid capacity is. It's a trade-off.

From what you're saying the prop valve has to be moved. It goes between the stock gutted prop valve and the rear line, so you will need to bend up a short line between the prop valves. That *should* get your pressure back and everything working properly. However, if it doesn't you may have collapsed soft brake lines on the rear end (hard lines-to-calipers). Those are notorious for breaking down and collapsing internally, thereby cutting off fluid flow to the calipers.

Another option, which some people have tried and reported to me works fine, is to use a regular, factory non-adjustable prop valve from a disc brake car (1987-88 Turbo Coupe, Lincoln Versailles, etc.) and skip the adjustable prop valve. The theory is that disc brake prop valves already have the metering set for the rear brake fluid flow. It's something like 70/30. This is something to consider (although it's probably too late for you now LOL)...being that I have not personally tried it, it's something that I cautiously recommend. It makes sense to me.

Let us know if the repositioning helps.

Thanks, I will move it. I am stumped as to why it won't work there.

Most of the time it's more how I read things, rather than what the text actually says, but I placed it there cause that's what I thought the write up said.

My rear soft lines have been replaced with the swap.

Thanks again for the advice. I'll report back what I find.

Eric,

By 70/30 you mean 70% front, 30% rear? If so that makes the ratio 30/70 = .428. Now that's the same ratio listed in the factory manual for the '86 brakes, 400 psi split point, .43 ratio to the rear after that. So why wouldn't just keeping the '86 (I'm assuming the 85.5 is the same) prop. valve work the same?

I was probably thinking of the factory prop valve there, sorry...brain freeze LOL. The rear disc prop valve should be in the neighborhood of 60/40, where the rear discs are doing more of the stopping. Whatever is listed for a 1987-88 Turbo Coupe should be about the same for most 1980's-early 1990's RWD rear disc factory cars (Lincoln Versailles, Mark VII, 1993 Cobra, etc.). So according to the people that have reported, that's the factory-style prop valve to get.

Does anyone have the '87 or '88 Ford Chassis/Electrical manual. It should list the proportioning valve specs. in the brake section. Also the fitting sizes might be listed.