1987 V30 Rear Brake drum to disk conversion

I purchased this pickup as a project and some things had already been half ass done. The Rear disks are evidently anothter partially done portion to add to my list.

I have no rear brakes... just found out driving on ice for the first time this year.

I have no pressure to the rear calipers.

Calipers looked like hell so I replaced them... But now I am learning that the master Cylinder needs to be replaced with a 1980 Corvette MC and there is a proportioning valve that needs to be replaced as well.

Ive tried the reset on the factory valve but nothing... I would think that the rear brakes would still work with the factory MC.. maybe not as well as they could but still.??

without starting over on this portion of the project...... any tips?? I ordered the MC today and am in the process of looking for the valve that will work. I was told a 4V adjustable porportioning valve... Anybody done this before?

I should add that this pickup started life as a 1987 1/2 ton...but the complete suspension and Front and rear end from what I was told an 1981 K30 was swapped. Dana 60 up front and a corp 12 bold in the ass end.

14 full float would be the rear axle of a k30.

you shouldn’t need to do the master cylinder. A disc/disc prop valve would be a good idea. Check also to see if you have a load sensing valve under the bed/over the rear end—it’ll be inline with the brake tube. Those can fail. Bypassing it is the fix.

When you bleed the system with the new prop valve, a brake bleeder vacuum can be helpful. Sometimes the prop valve can uncenter(as it’s designed to do with a loss of pressure on one side) and derail your bleeding progress.

I agree with the dicision to by a master a caliper requires a larger volume of fluid than a wheel cylinder to act. Also most master's are going to have residual pressure valves built into them 2lbs residual pressure for discs 10lbs for drums so if your master has built in residual pressure valves your rear brakes will drag.As for a proportioning valve I would use an aftermarket adjustable valve, because who knows what factory proportion valve is going to have just the right bias for your 79 Oldsmobuick front brakes, whatever rears and your 80 corvette master.IMHO just taking a proportioning valve off anything is going to be a roll of the success to how the bias works out.

Sorry I meant 14.... after rebuiding the king pins on that Dana 60 I think i might have brain damage....

Appreciate the info. I am doing the MC tomorrow and the valve as soon as it gets here... 3v does one to each front wheel and one to the rear wheels

4V does one line to each wheel... What is your guys opinion??

I wish he would have just left the DRUM brakes in it if he was only going to do it half ass. I hate half ass when the correct way was only 150 more... stupid to cut that corner especially with Brakes...

Greatly appreciate this group... Ive owned 13 Squares but finally have the one I always wanted...

When I did the rear brake conversion on mine I left the stock MC on it. Had rear brake lockup even on dry pavement. So I ordered the 60s corvette disc/disc MC. Still had rear brake lockup on dry pavement.
Then I ordered "The right stuff" PV72 disc/disc combination valve. Still had rear brake lockup on dry pavement.

Then I ordered an adjustable proportioning valve to be put inline on the single brake line to the rear to limit the pressure. That finally allows me to balance the system.

I’m not sure if I lucked out, or if my rear brakes just suck...lol

I used a disc/disc prop valve from summit that was the same size and shape as the original. Except the original had what I assume was the residual valve on the outlet port of the rear brakes. It was a cylinder shaped valve that threaded in-line between the prop valve and rear brake line.

Im not sure how the MC varies from vac boosted to hydro boosted or k10/20/30, but if my test is accurate, I don’t believe I have any residual pressure. I jacked the rear end up and both tires rotate easily. My MC is a stock replacement for an 86 K30.

77 K20 said:

When I did the rear brake conversion on mine I left the stock MC on it. Had rear brake lockup even on dry pavement. So I ordered the 60s corvette disc/disc MC. Still had rear brake lockup on dry pavement.
Then I ordered "The right stuff" PV72 disc/disc combination valve. Still had rear brake lockup on dry pavement.

Then I ordered an adjustable proportioning valve to be put inline on the single brake line to the rear to limit the pressure. That finally allows me to balance the system.

Click to expand...

Glad I read this. I will be installing my rear next weekend and jus had that thought. Luckily I do have an adjustable prop valve already but I will be using the stock disc/drum master for now. I need to look into using a master from an early 2000's truck that had disc/disc

I read about rear disc brake conversions for months before trying mine. I still don't see how a lot said just slap the rear discs on and keep everything stock as it will work perfect! Maybe my definition of perfect is different than others...

Or in the last 40 years some previous owners has changed out different parts of their brake systems.

Or I just have **** luck. Speaking of that "luck" I bought rear disc brackets from DIY4X and had problems with both of them. Their brackets have bends in it for the offset needed. Then they weld in another chunk of metal to make it a bit stronger. Either their bends were off a bit, or the heat from welding warped them as the brake caliper slide bolts when threaded into the brake bracket were not "shooting straight across" to line up with the hole on the other side of the brake caliper. One side was 2/3 of a hole off. This caused my inner brake pad on that side to wear down to nothing in less than 6,000 miles. The other side wasn't as bad, but still not good. And I had to use some very thin washers to shim out the brake bracket on the driver side as with new pads I couldn't get them to fit over the disc. (was too far off to one side- not centered).

Just a few weeks ago I finally got around to "doing it right". I ordered ORD's brake bracket setup. Just nice simple flat plate steel. Had zero issues with it. Wish I would have done it that way the first time.


Shims used.


My disc/disc combo valve and the proportional valve.

I did my rear disc conversion a couple months ago.Redid the whole brake system.

I had to bleed the brakes twice.(pain in the ass by yourself)

Rock Auto-Went with 1980 corvette master cylinder,rubber lines to front discs,rotors,and calipers.

Summit-pV4 combo valve.Made in USA.

ORD-14 bolt brake line hose kit (nice)
made in USA

Sky offroad-Disk conversion brackets (nice) made in USA

Inline tube-stainless steel hard line,pre bent-made in USA

overall could have done this conversion way cheaper and simpler.However,pulling a trailer and just needing to depend on the brakes,why screw around!

Working very well.Pedal is better than ever been.Happy with all parts.If you need pics or anymore info,reach out

I just opened my proportion valve from Summit any pointers on where to mount. it has a big knob sticking out of it to adjust pressure so the stock location under the radiator is out of the question..

I didn’t use a proportioning valve.That was the whole purpose of the combination PV4 valve from summit.

The oem combo valve is for front disks,rear drums.

The above mentioned aftermarket valve is for a disk and disk set up,as well as the master cylinder from a corvette.The corvette was the only GM vehicle during that period that had disk brakes all around.

These parts are 100% bolt on.The valve bolts to same place and no mods needed.

I believe if you use that proportioning valve,it has to go on after the PV4 valve to control line pressure to rear brakes.Maybe a good place would be on the frame rail where there’s a break in the steel line,about where the cab and bed meets.

Here’s a few pics

Thats the same Valve I purchased... but I got the adjustable one. it replaces my factory one but dont think it will fit .. adjustment handle will hit fan shrowd. I want to switch to electric fans but didnt want to do it today. lol..

well.... im still stuck

new rotors front and rear. new booster.. new 1980 corvette MC.. adjustable proportioning valve.

brakes are hard when not running... as soon as I start it they go to the floor. bled with phoenix brake bleeding system as well as 100 times traditional way... I just dont get whats going on...

The most frustrated Ive been since I installed transmission cooler lines on the wifes 2008 jeep overland grand cherokee with a 5.7 hemi wedged in it...

This brake system is so simple but I cannot get it to work correctly...

had a vacume leak under TBI... fixed.. great vacume at the MC... only guess I ahve is a faulty new part that I have replaced... everything is new except the short lines to each wheel. they look good... but since Im this deep I may as well replace them as well. Any ideas

I did this conversion in July.Didn’t use a adjustable valve.Replaced the combination valve on the front c.m with a disk-disk pv4 combo from summit.Did the same m.c. and 4 new calipers and rotors from rock auto.New brake lines...all of it.Had the same issues as you described.Kept driving short distances,then bleed.Using a temp gun on each disk.Trying to figure out where the air was.It got better and better until I finally got all the air out.I was very frustrated and was regretting doing the swap for a minute.Saw other people online saying the same things.Just stay after it.

I also saw where a guy found the pin wasn’t able to push the m.c. correctly off his booster.He ended up using emery cloth to get full engagement and it fixed his problems.Hopefully you’ll get that thing figured out soon.There was one or two vids on YouTube that was actually helpful out of all the rear disk conversions crap on there.