disc brakes in rear

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ptodave390
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disc brakes in rear

Postby ptodave390 » Tue Apr 07, 2009 9:07 pm

Have a 72 4x4 with 14 bolt rear,we just put disc in the rear. Used all chevy brake parts. The M/C is off 79 f-350. Truck has a HP 60 in front. Brakes work OK but could be better. Any ideas how to make rear disc work better? Ended up with GM 14 bolt because rear 60 broke axles all the time. Truck weighs 7430 pounds. The 78 2WD 14 bolt bolted right in.Thanks PTO.

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Re: disc brakes in rear

Postby FORDification » Tue Apr 07, 2009 10:01 pm

Are your rear brakes still hooked up to the factory proportioning valve? If so, that's probably your problem. With a disc/disc setup you can't use the proportioning valve, which is designed to hold off the application of the rear brakes momentarily upon application and to limit the pressure to the rear circuit. You'll probably want to install an inline manually-adjustable valve in the rear circuit though, to fine-tune things.
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Re: disc brakes in rear

Postby willysrul » Wed Apr 08, 2009 6:13 am

x2 :yt:
67 HiBoy - 390/435/TS'd205 HPD60f/FFD60r welded 3.54's 36x12.50x16 TSL radials
50 Willys P/U (project) - 302/435/205 FFD60r

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Re: disc brakes in rear

Postby averagef250 » Wed Apr 08, 2009 11:24 am

You can't just bolt on rear discs. The right way to do it is get the appropriate master cylinder for the application. To do this you need to do some math based on the bias you want, the CC's of the front and rear brakes and the bores of the master cylinder. The adjustable PV is a bandaid of sorts that will get so-so results.

I feel retrofit rear discs never, ever work right unless you totally re-engineer the whole nine and are mostly a waste of money unless you just putt around empty or offroad. If you really want rear discs run the whole factory setup off a late model truck. Rear discs are rear discs, they are not fronts put in the rear.

3.5" 1 ton rear drums will stop 20K pounds in short order. Mismatched rear discs will just melt.
1970 F-250 4x4 original Willock swivel frame chassis '93 5.9 Cummins/Getrag/NP205/HP60/D70

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Re: disc brakes in rear

Postby mr_josh » Wed Apr 08, 2009 3:07 pm

averagef250 wrote:You can't just bolt on rear discs. The right way to do it is get the appropriate master cylinder for the application. To do this you need to do some math based on the bias you want, the CC's of the front and rear brakes and the bores of the master cylinder. The adjustable PV is a bandaid of sorts that will get so-so results.

I feel retrofit rear discs never, ever work right unless you totally re-engineer the whole nine and are mostly a waste of money unless you just putt around empty or offroad. If you really want rear discs run the whole factory setup off a late model truck. Rear discs are rear discs, they are not fronts put in the rear.

3.5" 1 ton rear drums will stop 20K pounds in short order. Mismatched rear discs will just melt.


Well-said, sir. There is something of a misconception that as long as the calipers will bolt up, and as long as the plumbing connects, any brake part will work anywhere.
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Re: disc brakes in rear

Postby ptodave390 » Wed Apr 08, 2009 9:04 pm

Maybe you guys could help me with the math. 79 f350 dayton dual calipers in front, 84 k20 2WD calipers in rear. Maybe a different M/C? Or can I run rear line direct to M/C? With PV? By the way the truck stopped VERY GOOD with 13 by 3 and a half inch drums. Truck sees a lot of mud and thought it would better for me to run discs. Thanks PTO. :roll:

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Re: disc brakes in rear

Postby averagef250 » Mon Apr 13, 2009 4:45 pm

You will have to actually measure the bores of all your calipers. Pop the pistons out and take a mic to them. The big issue you and every other person who falls under the misconception 70's GM front calipers belong under the rear of their vehicle has to deal with is these are not REAR calipers. There was never a fullsize truck application that used these in the rear so you cannot obtain what an OE application would have used for a master cylinder bore to work with calipers and what the apprpriate front caliper piston area should be to match.

If you just hook up the GM calipers you have to what's already there you'll have a truck that stops fine empty in an average stopping situation. What you won't have is a truck that stops well in a panic stop or while slowing a load down a grade.

I've had the exact setup you have there under my '70. It never worked well, I switched it back to 1 ton drums after anhialating the rotors and melting the calipers twice towing at the legal limit.

I have a customer right now who's 80's GM K30 I'm installing a Cummins, 5 speed, and regearing the diffs to 3.73 with ARB's from 4.56's and a rear detroit. He has his heart set on rear discs and refuses to listen to anything anyone other than people trying to sell rear disc brackets have to say. I know he's going to hate the discs and want something different as soon as the thing's on the road.

I've been thinking about going to rear discs again under my 70 because I'm going to run a late model sterling under the rear instead of the D70 and all metric sterlings are disc so I may be able to incorporate discs. The deciding factor for me is going to be taking measurements off a pair of late model F-350 axles and the trucks master cylinder to see exactly what the caliper piston surface area is and what the MC bore is to match. IF and only IF I can adapt sterling discs to a non-metric sterling without machining entirely new rear hubs and if the unit-bearing hub 60 fronts use the same caliper piston surface area as the older 77.5 through 97 F-350 pistons then I will consider the swap.

I'm perfectly happy with rear drums, but figuring out how to do rear ford discs and have them work right would be interesting. My 89 7.3 F-350 4x4 long chassis 11K empty truck will stop itself and a 10K pound trailer in tow without breaks under 45 MPH on a dime with it's drums/discs. I've been blown away by how well the brakes work on that truck, I think they are the "superduty" front calipers that have extra large pistons in them. I haven't even looked to see how big the drums are in the rear.

Also, if you go in mud and want something that's easy to clean and isn't disc get an 85-97 sterling. The drums fall off when you pop the wheel off.
1970 F-250 4x4 original Willock swivel frame chassis '93 5.9 Cummins/Getrag/NP205/HP60/D70

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Re: disc brakes in rear

Postby ptodave390 » Mon Apr 13, 2009 8:27 pm

Had a chance to get the fall off rear brake drums off a 14 bolt, maybe that might have been a better choice? :roll: You described perfectly how the truck is now, stops ok empty,but I am not sure what would happen in a panic. Thank you very much for the info, I will try disc for a while,could always go back. :roll:


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