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Old 11-15-2020, 11:27 PM   #1
JMD
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Rear brakes lock up before fronts

Hey all,
I did a rear disc conversion on an HO52 axle for my ‘68 k10. The front axle is a Dana 44 from a mid ‘70s truck that also has disks. I used front calipers from a ‘75 k20 on all four corners.
The proportioning valve is made for a 4 wheel disk conversion and has 1/2” fittings for the rear and 3/8” fittings for the front.
The brakes work fine, but when I really have to stop quickly the rear tires lock up way before the front. In fact, I don’t think I can get the front tires to lock up at all. And worse yet...I can’t even power brake the thing because the rear brakes grab so hard.
I attached a picture of the way I have everything plumbed. I know I can insert an adjustable proportioning valve to reduce the amount of braking power to the rear, but that’s not really the “correct” way of doing things. So what combination of parts do I need to fix this annoying issue?
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Old 11-16-2020, 02:11 PM   #2
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Re: Rear brakes lock up before fronts

Did you use a Corvette master and proportioning valve?

On my Camaro I used C10 calipers front, and rear. Plus I used 68 Corvette break parts.

On my instructions the front was the front, and the rear was the rear on my master/ proportioning valve. I don't know what backwards would do, but maybe the master applying for fluid volume to the back
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Old 11-17-2020, 01:05 PM   #3
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Re: Rear brakes lock up before fronts

Anything 'non-stock' is going to require an adjustable valve to dial in proportioning.
The fronts do 70% of the braking. If both ends receive equal fluid volume & pressure, I would expect the rear to lock up first.
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Old 11-17-2020, 02:24 PM   #4
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Re: Rear brakes lock up before fronts

Thanks guys,
I did some research and I was under the impression that an adjustable proportioning valve was simply placed in line with the rear brakes in addition to the distribution block I already have. Apparently the adjustable valve replaces the distribution block entirely. I’m guessing that will solve my issue.

The master cylinder and booster I have are just a summit brand kit. I can’t seem to find the part number anymore but I believe it was designed to be used for 4 wheel disk applications. The only reason I criss crossed the lines from the master cylinder to the distribution block is because that’s how the fitting sizes lined up. If I ran them straight across I’d have to adapt the 1/4” fitting to 3/8” and vice verse.
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Old 11-17-2020, 02:54 PM   #5
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Re: Rear brakes lock up before fronts

Quote:
Originally Posted by JMD View Post
Thanks guys,
I did some research and I was under the impression that an adjustable proportioning valve was simply placed in line with the rear brakes in addition to the distribution block I already have. Apparently the adjustable valve replaces the distribution block entirely. I’m guessing that will solve my issue.

The master cylinder and booster I have are just a summit brand kit. I can’t seem to find the part number anymore but I believe it was designed to be used for 4 wheel disk applications. The only reason I criss crossed the lines from the master cylinder to the distribution block is because that’s how the fitting sizes lined up. If I ran them straight across I’d have to adapt the 1/4” fitting to 3/8” and vice verse.
It's my understanding the master cylinder front to rear thing is volume related. On disc/drum set-ups, it's common to have more volume for the front discs. Master cylinders like what you're using have equal size chambers & since you now have 4-wheel disc it's a moot issue.

The only consideration is the actual prop valve. If it's an OE style disc/drum prop valve, there's prob an advantage to changing to a proper disc/disc unit (spec'd per model) or an adjustable/universal unit. Adjustable units are also the proper choice when utilizing larger diameter tires in the rear vs the front.
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Old 11-17-2020, 02:55 PM   #6
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Re: Rear brakes lock up before fronts

Looking at how you have your lines run , it looks like you have the front of the master cyl going to the rear brakes....your moving a lot more fluid to the rear instead of the front
I used a POL unit on mind..the front of the master goes to the front brakes..the rear goes to the rear
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Old 11-17-2020, 04:52 PM   #7
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Re: Rear brakes lock up before fronts

My understanding was that the larger brake line ran to the rear brakes since it’s a longer run. Is that not correct?

What’s the purpose of having a proportioning valve with 3 outlets with one outlet being bigger than the other two?
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Old 11-17-2020, 05:25 PM   #8
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Re: Rear brakes lock up before fronts

On my POL prop valve, there are 3 outlets...1 for the rear and 2 for the front..the 2 front fittings give you the option to run a single line to a splitter with 1 opening plugged,.... or 2 lines from the prop valve , 1 to each front wheel
I opted to plug 1 outlet and run a single line to a splitter in the center of the frame under the radiator,,,and then on to each front wheel...I basically copied the brake system that was on the burb I parted out for the axles
All my front lines are 3/16, my rear is 1/4 that splits in to 2 3/16 lines on the rear axle..
Mine is disc drum
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Old 11-17-2020, 05:33 PM   #9
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Re: Rear brakes lock up before fronts

If I read the original post correctly you installed front calipers on the rear? Typically the front calipers are quite a bit bigger than the front. Front calipers have larger pistons for more braking pressure. I think that's your problem. The front calipers on the rear are applying way too much pressure. You can try an adjustable prop valve but I don't think will work.
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Old 11-17-2020, 05:34 PM   #10
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Re: Rear brakes lock up before fronts

I could be wrong ,but I think that your putting your high volume to the rear brakes instead of the fronts..
Like I stated above, dont just go by the tubing nut size as there are multiple thread sizes for the same size tube..
What brand is your prop valve and did it come with instructions?
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Old 11-17-2020, 06:23 PM   #11
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Re: Rear brakes lock up before fronts

Quote:
Originally Posted by LS short box View Post
If I read the original post correctly you installed front calipers on the rear? Typically the front calipers are quite a bit bigger than the front. Front calipers have larger pistons for more braking pressure. I think that's your problem. The front calipers on the rear are applying way too much pressure. You can try an adjustable prop valve but I don't think will work.
^^This^^

Mongocanfly might be on to something w/the line sizes but your master cyl is equal size (not a stock replacement) so the line fittings connect where they fit. I could see an issue if you bought everything from separate sources & were trying to piece parts together (lines from source #1; master cyl from source #2; prop valve from source #3) but it seems you're working w/a kit of some sort.

A disc/disc set-up has proportionally smaller piston sizes for the rears on OE set-ups & even most aftermarket kits. If your set-up is utilizing D52 calipers front & rear, you're going to need to adjust the hydraulics to dial things in.
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Old 11-17-2020, 09:48 PM   #12
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Re: Rear brakes lock up before fronts

I did some digging and found the exact parts I have installed. The master cylinder/booster kit is indeed meant for a front disk, rear drum setup. Even though summit doesn’t mention anywhere that this is the case, it seems to be the general consensus in the product FAQs.
https://www.summitracing.com/parts/sum-760197


The proportioning valve I have is made for a disk/disk setup. I think when I bought these parts 6 years ago my idea was that I could use this proportioning valve to correct the fact that the master cylinder is meant for a disk/drum setup.
https://www.summitracing.com/parts/sum-760185

So maybe switching the two lines going in to the proportioning valve will fix it. Or getting rid of the disk/disk valve and getting an adjustable one.

An adjustable valve only affects the rear brakes, correct? How exactly does that work?
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Old 11-17-2020, 10:20 PM   #13
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Re: Rear brakes lock up before fronts

I always run a adjustable prop valve with my disc/disc swaps because I don't typically build anything stock.
I'm working from memory so that's more than a bit scary but I seem to remember that an adjustable prop valve doesn't limit the pressure to the rear just slows the application down some how or another. I use Wilwood prop valves. Just the simple one that has one line in and out for the rear line. Not the best pic but you can see the prop valve below the master.
Again I'm far from a brake expert but with your combo I would delete the stock prop/combo valve.
So my go to guy on brake systems is Tobin at KORE3.com. Call him and tell him what you got and he will tell you what you need. You might not like what he will have to say but he a brake expert.
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Old 11-17-2020, 11:00 PM   #14
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Re: Rear brakes lock up before fronts

the adj valve goes only in the brake line going to the rear brakes
this explains it pretty good.. https://www.youtube.com/watch?v=DbSwo_x-ozE
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Old 11-20-2020, 01:14 AM   #15
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Re: Rear brakes lock up before fronts

Foward most master cylinder port is going to the correct location on the prop valve
.. the rear... (in your picturesy) as long as the rear brake line is the one coming from the center rear of the prop valve ypur set, juhhst need the adjustable valve
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Old 11-20-2020, 09:11 AM   #16
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Re: Rear brakes lock up before fronts

I tried to do some research into the parts you got from summit..
Heres the instuctions for the master you have...and the prop valve you have..
Notice on the master cyl the line positions going to the prop valve...look closely at the nut size on the lines at the prop valve ..you can visibly see the large line nut I referred to before..
Also the line size is the same..front and back between the master and prop..

Going by all this ,I still stand by that its hooked up backwards....
I can always be wrong though..
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Old 11-20-2020, 09:14 AM   #17
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Re: Rear brakes lock up before fronts

And the prop valve
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Old 11-20-2020, 09:24 AM   #18
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Re: Rear brakes lock up before fronts

Also , if you didnt use the little tool to hold the prop valve spool centered while bleeding, then it could be tripped..
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