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Disc Brakes on a BOP Differential

 

For the disc brake swap, you will need the parts off the following cars:


-4 wheel disc Firebird T/A or '75-'79 RWD Seville (K-car)

Rear disc calipers

Caliper brackets 

 

-4 wheel disc Firebird T/A

Rotors 

Combination valve

Master Cylinder

 

-Other (Fabricated)
Plumbing lines from the rear differential "T" to the calipers; steel or steel braided
Steel brake tubing or steel braided line
Tubing adapters (maybe)

 

-If you wish to make your own hard lines:

A tube flaring tool that can create a double-flare

 

 

As for the parts, the most important part to find will be the combination valve. The combination valve contains a proportioning valve which is what balances the braking front to rear, and also contains a metering valve that will not allow the front brakes to start applying until adequate pressure has been sent to the rear brakes. Just remember Combination valve = Metering valve + Proportioning valve. Without this the front will nosedive when you hit the brakes, or just plain lock up unexpectedly. Some have said you can run without a combination valve. In my book that is absolutely false, take it or leave it. The engineers put the combination valve in there for a reason and most of them are not around any more to argue the point with, so I will just have to trust them. Just know that many cars equipped with a drum/drum braking system used nothing more than a brass block at the end of the hard lines coming from the master cylinder. All this this block accomplishes is really nothing more than a distribution block in the lines so to split the front brakes left to right. An earlier car of the same vintage that had the option of front disc brakes (a disc/drum system) may retain this distribution block, and a small round cast iron valve would have been included in the hydraulic system, usually just below the master cylinder.

 

As for acquiring a combination valve you can either get one new, or from a 4 wheel disc T/A in the junkyard. Spend the $50 and never have to think about it again.

 

The combination valve is attached to a bracket hanging from the master cylinder mounting bolts and has hard lines that come straight from the master to it. From there, the hard lines come out and go to each front wheel, and one large line goes to the rear brakes. Unfortunately, a disc/disc and a disc/drum combination valve look almost identical. You must get the combination valve (new or junkyard) for a 4 wheel disc car! If you inadvertently install one from a disc/drum car, you could possibly destroy the new rear brakes within a few hundred miles and/or have an improperly balanced braking system. The disc/drum combination valve was meant to be coupled to a pair of rear wheel cylinders of +/- 7/8 inch in diameter. Your new rear calipers are 2-1/2 inches in diameter, and the disc/disc combination valve was built with this in mind. You can also add a 10lb. brake pressure residual valve from Jegs or Summit Racing and plumb it into the rear line between the combination valve and the rear end. It will leave a little pressure in the rear lines (without forcing the pads into the rotors) and give you a higher brake pedal.

 

Get a new master cylinder for a 4 wheel disc T/A to use on your car. 

(Note: You can even use an aluminum master cylinder from a pre-'91 Chevy Caprice. See the next article for that.)

 

Mount the master and the combination valve to the booster, and plumb the brakes. You may be able to use the hard lines to the front brakes with some tweaking with a tubing bender, and maybe an adapter. As for the rear line, you can extend the original line (where it went into the old combination valve) with a new section of steel line or braided line and adapt it on, or run one long piece of new braided steel brake line to the "T" at the top center of the rear end. Then plumb hard lines from the master cylinder to the combination valve. There are industrial hose or racers supply houses that can make (and test) braided lines to your specs. 

 

The only thing left to fabricate is a pair of shims for the BOP axles. You see, the original drum brake backing plate is 3/32" thick. The disc brake backing plate is 3/16" thick. Since you bolt the backing plate between the axle retainer plate and the housing, without shims, the axles will be able to pull in and out of the housing the extra 3/32" (0.09375"). I also discarded the original paper gaskets since mine were trashed, and made new ones from gasket material. Those were 0.060" thick, so I added that to the total, for about a 0.150" (one-hundred fifty thousandths) shim needed. Measure your gaskets, or remake yours and just go with the 0.09375" measurement, plus the thickness of your new gaskets.

 

I used Chevy 7.2" rear end shims to fabricate the ones I needed. They're ground flat, at the thickness I needed, and most importantly - ROUND! A few minutes with the bench grinder to make the outside I.D. fit just inside the axle housing is all that was needed. The best part is, you can get packs with different thicknesses to match your needs.

 

With that done, the rest is easy. Go to the junkyard and find a '75 to '79 Cadillac Seville rear wheel drive or 4 wheel disc Firebird T/A. Those year Seville's had FWD and RWD versions. The correct car is a RWD, and it is actually a Chevy 10 bolt c-clip rear. They will be much easier to find, and will be cheaper too. Get the caliper brackets and the tin shields. You'll have to pull the axles for that, and then remove the 4 bolts holding on the brackets on each side. Keep the bolts, and note the right and left bracket. Also note the position of the calipers. They are not equal on both sides. As for the calipers, I suggest getting new (rebuilt) ones from a parts store. They are really difficult to rebuild yourself if you don't know what you're doing. They will be pretty cheap from the parts store and warranted too. Also get the parking brake cables from the junkyard car.

 

Just a quick note: try to get the calipers from a '79 car. There was a small design change to the caliper piston in '79, and it does work better on the Pontiac 10 bolt rear.

 

Now, just transplant all the pieces onto your rear. As for the caliper hard lines, I had a set of braided lines made that go from the "T" fitting to each caliper, with enough extra length to create a loop. Without the loop, you won't be able to remove the calipers for pad swaps. You'll see what I mean when you get the parts from the junkyard car.

 

As for the rotors, the T/A uses the same rotor as the Seville, but the Seville bolt pattern won't match, so you'll have to get a set of rear rotors for a T/A. You will need to have the axle flanges machined to fit inside the rotors.

 

Adapt the parking brake cables to the original setup, and adjust the cables to get just a half of a hint of drag on the rotors from the pads. Use of the parking brake is mandatory!!! It's the mechanism that keeps the brakes in adjustment.

 

Just bleed the setup and you are done!

More Mods For Even Better Brakes

Want more? OK!

 

While you are at the junkyard getting the parts for the rear disc swap, find an aluminum master cylinder from a pre-'91 Chevy Caprice or wagon. --It does not matter that the donor car has rear drums. The residual pressure valve usually in the drum brake side of the master cylinder that would normally destroy the disc brakes is elsewhere in this braking system.-- Also, find a '78-'87 GM G-body (LeMans, Cutlass, Malibu, Skylark) and swipe the dual-diaphragm vacuum booster.

 

For the master cylinder, all you have to do is mount it like any other except you need to remember that the brake line ports are backwards. The larger piston is in the back of the master cylinder, (closest to the firewall), and thus controls the front brakes. Just setup your brake tubing accordingly.

 

For the vacuum booster, you need to modify the pedal shaft and change the firewall brackets. First, measure the distance from the centerline of the clevis hole (the part that attaches the bracket to the brake pedal) to a marked point on the the firewall bracket. Write down this length for later. Then, remove the bracket from the A-body booster that attaches the pedal to the booster shaft. Remove the same bracket from the new booster with a hacksaw, leaving as much of the shaft as possible. Cut threads into the shaft to match the threads on the A-body booster. I believe it's 5/16-18, but double check it first.

 

After cutting the threads, use a shock absorber extension bolt, (meant for 4x4 trucks - the threads match the standard A-body front shock upper shaft - again I think it's 5/16-18), thread a lock nut and the shock extender onto the new booster shaft with lots of Loctite and use the extra nut to lock it down. Thread the pedal bracket from the A-body booster onto the shaft along with another locknut, and lock that down too after verifying the correct length after attaching the A-body booster firewall brackets.

 

For the firewall brackets, all you need is to cut or grind the rivets that hold the firewall brackets to the A-body booster, enlarge the holes a tad, and bolt it to the new booster where the old brackets were. Bolt the new booster assembly to the firewall, put the clevis pin through the bracket and pedal, and you're done.

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The new motor/trans/exhaust/fuel system is in!

Best run so far: 12.38 @ 109.24

462 cid, 6x-4 heads, 250 cfm

Cam specs:

Intake/Exhaust
@ .006 285/290
@ .050 235/240
Lobe Lift .338/.331
Gross Lift @ 1.5:1 .507/.496
Lobe Separation 106 Deg
Installed Intake Centerline 102 Deg.

TH400 Trans built by

Hubbe's Performance

 

http://hubbes.com/

 

Continental 'Tight-10 inch' converter.

 

3.36:1 posi Pontiac 10-bolt rear

Tank with sump and baffle welded in.  Bungs accept NPT to -10 AN fittings.

 

140 GPH pump and filter, braided lines throughout.

Doug's headers

 

Pypes 3" exhaust with X pipe, 18" Goerlich XLelerator mufflers, and 3" tailpipes.

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Thanks to Mr. Jon Burchmore for the creation of this file, and the permission to post it here.

 

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