Proportioning valve question

[black_k5]

Hey guys I have a question on my brakes. Last year I swapped my axles for a set of 3/4 tons out of a 86 suburban. These have the optional larger rear drums, I think 13". When I brake my brakes light comes on and I have poor braking since I assume the piston shifts and cuts off the pressure off to the rear. I was told that the new wheel cylinders are larger and take more fluid which causes this. They suggested adding an adjustable restriction to the rear line but I'm not sure that will work with out removing the factory prop valve and replacing it with a distribution block. The other option it to go back to the 1/2 ton axles. Is there much advantage to keeping the 14bolt semi float? Thanks guys

[truckster]

How do you use your truck? What engine/transmission combo do you have?

For a mostly-stock truck that's primarily driven on the street, the 12 bolt is more than adequate.

[black_k5]

So my blazer now has a 5.3, 4l60e, np205 and 35s on 20 inch rims. I do not off road it but I want to use it to pull my car trailer and eventually I'm hoping around a 24ft boat. Currently both axles have 3.73 gears which I was hoping to change out this winter for 4.10 or 4.56 gears and add a posi.I need to figure out which axles I'm going to use first and this brake thing has me leaning back to the originals. Also I do like to take it on road trips. So far I've done a few 3hr trips but hoping to take it on an 8hr road trip

[truckster]

I'm sure others who are more experienced than me will chime in, but in my experience the Blazer's short wheelbase makes it less than ideal for pulling trailers that large.

I had a set of axles from a 1-ton but decided not to install them because I don't offroad very hard. I think the main reason for going with the 14 bolt is to get the heavier axles as a hedge against breaking axles off-road.

[1rippen6]

Years ago I put a Dana 60 and 14 bolt under my 71 blazer and I installed a master cylinder and boost from a 1 ton truck of the same or close vintage. It bolted to my booster bracket and I never had any problems with the brakes that I can remember but were talking 25 years ago.

[hemi43]

You are correct, the newer rear axle has a larger piston area. That truck you got that differential from also has larger piston area on the front calipers which is why late model trucks have a such a large bore on the newer master cylinders.
You can't just throw that newer diff in and expect it to work using the stock master cylinder because it wont give you enough volume.
I'm also running a 5.3/ 4L60E combo, and I updated my front calipers when I did the swap. I'm using 2003 dual piston calipers but had to also use the 2003 master cylinder because it has a 1 1/4" bore and more stroke than the stock one, which I think is only around 15/16".
As far as the proportioning valve, all it does is reduce initial shock to the rear brakes in a panic stop so that your back wheels don't lock up. It does not change pressure to the rear. Another thing that valve does is it monitor brake pressure on front and back. If it notices and difference, it will shuttle a valve and activate a switch which will in turn put your brake warning lamp on on your dash. This is what's happening with your truck.

You're kinda stuck now because you have mismatched brake components. Your rear brakes require the volume of a large bore Master cylinder, but your front brakes require a smaller bore( higher pressure, less volume)

If it was me, I would try and fit smaller bore wheel cylinders on the rear. A lot of these parts are interchangeable.

Edit; I just checked NAPA online, and the bore of the 1986 wheel cylinder is 1 1/16" . The bore of the 1972 is 1". Not a huge difference, but probably enough to screw things up. I would check to see if by chance the 1972 wheel cylinder from your old diff fits the newer 1986 from the Suburban.

Edit #2; 1986 Suburban with 11" brakes uses a 1" bore wheel cylinder, and I would bet that it's a direct bolt in. For less than $10 each I would be trying this route first. IMHO

[black_k5]

Thanks hemi, I did not realize that even the master cylinder needs to match. Know knowing this I'm think I'm going to go back to the 1/2 ton axles. As long as the bearings can handle the 35's on 20inch rims then I don't see the point in using the 3/4 ton axles.

Hemi if I remember right your running 410 gears with 33s, would you suggest 456 for 35s for a good mix of around town driving and highway trips? Looking for the best gas milage

[Mike C]

I have the same setup in my Jimmy. I swapped my 12 bolt parking brake cables into the semi-float 14 bolt backing plates with the 13" brakes. The rest of the parts were from the Suburban, and 86 in my case.

On the front I kept the 1/2 ton calipers and I kept the 1/2 ton master cylinder. The 3/4 ton master swaps reservoirs front to rear compared to the 1/2 ton setup.

My brakes work awesome with it setup that way. In fact, we are doing the exact same swap into my son's 72 Jimmy we are building using a donor 84 sub.

[hemi43]

Originally Posted by black_k5

Thanks hemi, I did not realize that even the master cylinder needs to match. Know knowing this I'm think I'm going to go back to the 1/2 ton axles. As long as the bearings can handle the 35's on 20inch rims then I don't see the point in using the 3/4 ton axles.

Hemi if I remember right your running 410 gears with 33s, would you suggest 456 for 35s for a good mix of around town driving and highway trips? Looking for the best gas milage

With 35's I would definitely be running 4.56:1
That will put you just over 1800 rpm @ 60 MPH which is perfect. If you were to use 4.10:1, the engine would lug and you'd most likely end up with worse mileage.
That 12 bolt will be more than adequate. The only advantage to going to a bigger diff is for carrying heavier loads.

FWIW, I get 25 MPG (imperial) with my Blazer. Picture was taken past summer. 70MPH and reving just over 2000 RPM.

[black_k5]

That's awesome, I could only get 15.5 on my trips but I was loosing power in od around 70. My goal was 18 so now I know it's possible. Thank you

[black_k5]

Hey Mike somehow I missed your post yesterday. That is a very good idea. I may try that when I'm get a warm day and then I will already have new calipers when I rebuild no 1/2 ton axles. Thanks for the info