Rear Disc Brakes on a 70 Stage?

I've got front power disc brakes already and a 12-bolt out back. I've found several kits and short of replacing all my brakes, I'm not 100% sure what will be the most effective method. I'm left with a question or two.

I'd prefer something with the internal drums rather than the screw calipers, like this:

http://www.performanceonline.com/store/customer/product.php?productid=19545&cat=546&page=1

What else do I need to change on a '70 power disc brake car? Do I need a new master cylinder? I'm thinking I need a different distribution block/proportioning valve. Will I still need an adjustable proportioning valve?

I'll take pictures once I get figured everything figured out and document it if anyone is interested.

 

www.buickperformance.com
George has a good article on rear disc set-up. Late model Cadillac parts if I remember right. Might help you out. Friend of mine did this conversion on his '35 Chev/ Vette rod just after George posted the info.. Works like a charm.

 

Hmm, I've read those and one talks about the residual valve for drum brakes being a problem for a F0rd but I didn't really see anything that talked about a different master cylinder. The part about the proportioning valve being different in respect to the line and fittings is a little off-putting.

I also don't want to mess with the ratcheting parking brake. It sound problematic and there are a few horror stories out there.

I don't have access (or the time) to scrounge the junk yards any more. Funny how real life screws with your hobbies.

I was hoping to find someone that had actually done this and had some first hand knowledge, possibly answer the master cylinder/proportioning valve questions.

 

Wow, i didn't know about the a rear disc with drum park brake thats fairly cool. I've got a 4 wheel disc kit from Baer on a 12 bolt and got the new master cyl / proportioning valve with the kit. I may have bought this from Matts classic bowties? Theres also a kit from CCP available also and probably many more manufacturers?
I can post pics of my setup if you want, i didn't have the means to be looking in wrecking yards either, Gord

 

Only thing I could offer is I put rear discs on my 70 Stage 1 but I have a 9" in my car. But just for reference here is what I did as far as system design.
The aluminum bodied master cylinder is from an 82 LeSabre. The M/C piston is stepped for two sizes. I think the larger diameter rear portion of the piston is around 1 1/2" and the smaller forward one is around 1 1/8". I could be off on those sizes, its been a few years. Anyway the M/C was originally fitted for a front disc/rear drum system. In the body of the M/C on the drum side is a residual valve where the line screws into the body. I simply removed that valve. And installed an adjustable Kelsey Hayes proportional valve going to the rear brakes. The plastic reservoir for the brake fluid is a press in unit as found on most all newer cars. They vary in capacity and height and if they're favored more towards the left or right. I grabbed several reservoirs in a wrecking yard and found one that cleared the inner framework of the steel hood. I used an 82 Cavilier power booster and bolted it to the firewall after I robbed the brackets from a stock power booster unit and installed them on the Cavilier booster. Its much smaller in diameter so it clears the valve cover with plenty of room. The master cyl. bolted right up to the booster. Only thing I had to make was the pushrod that goes between the two units and the rod that attaches to the brake pedal. Then just made up new steel lines going from the M/C and prop. valve down to the distribution block on the frame. I used stainless braided teflon lines for each caliper. They don't expand as much as rubber lines do. Took a few tries to get it adjusted but it stops better than I could have imagined. Dang near gives you a nose bleed if you hammer the brakes. And with the smaller capacity of the booster it charges up quick for repeated stops in traffic.

 

That's good information. I just need to find out where the residual pressure is provided in the stock '70 setup. I've got one of those extra valves they did before they went to a one piece master cylinder without it. I'm hoping that is what I'm needing to remove if I can find out for sure what it does.

Then I find stuff like this at Inlinetube.com and it makes my head hurt! I've never made brake lines and am a little uncomfortable making double flared ends. I'd much rather do mechanics than hydraulics.

GM Notes:

1967-70 cars all came standard with a dual master cylinder and drum brakes, and 1967 was the first year for optional disc brakes. If the car had either drum or disc brakes, there was a distribution block with a built-in brake warning switch down on the frame below the master cylinder. From 1967-69, if the car had factory disc brakes, it used the drum distribution block in conjunction with a disc brake hold-off valve, which made the disc cars a 2 part valve system. In 1970, GM added a third valve in the rear line for brake correction, so the 1970 system had three valves. For cars converting to disc brakes, replacement aftermarket 1 piece valves are also available. Conversion valves will not plumb into existing factory lines.

 

The hold off valve is what's hanging off the outboard side of the M/C. Its used to delay the application of the rear drum brakes until the fronts could be activated. To prevent rear brake lockup. The residual valve is most likely in the body of the M/C. A quick phone call to inline tube would answer that.

With the proper tools making up the steel lines is pretty straightforward. You'll need a good quality double flare tool kit and a good quality tubing bender. Get some steel brake line and practice on it before you make your lines. Here's a couple shots of the system on my car. I need to remake the bracket that holds the adj. prop. valve, the current one is pretty funky.

 

I went with the TSM kit and found it very easy to install. I also used the stock front disc proportioning valve with no problem so far.

 

I'd have to refresh my memory on the way the hold off works. Its been years since I've read about it. There is a description in the 70 Manual (the big red book). I'm at work right now so no book available. But I thought it was to keep the rears from applying too soon to prevent the rear from locking up. I guess it'll be time to break out the book when I get home tomorrow.
I omitted that valve however with my new system.

 

Yea, I forgot to mention my car is a 1971. It does not use a hold off valve like in 1970. Just a proportioning valve mounted on the frame under the car. That could be why mine works ok. They DO sell a Disc/Disc, 4 wheel Disc proportioning valve for conversions.

 

But everything I can find says they are not a bolt in. I'm not sure I can get to it and replace it, let alone reflare the ends without yanking the motor. I'm about half tempted to pull the engine any way since the thing leaks around the pan something fierce.

Curiouser and curiouser. Mine has the small line in front with the little extra valve that looks like it is the front disc brakes. The big line is in the back and seems to go to the rears. The little external valve is just a pass through, there is not a third fitting so whatever it does is just on one line. It looks like this one:

http://www.piratejack.net/Proportioning-Valves/PV68-Camaro-1967-69-Proportioning-Valve-Bracket.html

I suppose I'm going to have to study some brake technology pretty hard before I can make any choices... or I can slap discs on the rear and see what happens. What's the worst case scenario? It locks the rears pretty easily?

 

Yes, you're correct. combination valve. My bad. As for his Tubing what will he need to do for the conversion ? Do they sell the modified valves for the 1970 ? Or will he possibly need to go with new tubing from, Inline Tube ? Or change over to the 1971 combination valve and tubing ? I'm sure TSM sells the combination valve and some other vendors also.

 

I totaled my other toy, an '04 Mazdaspeed Miata, and put my wife in a halo from a roll-over. I think I'm going to catalog this information for future use. Right now, non-required improvements are on hold.

Valuable information. I'm hoping insurance covers the cost of the car AND all the medical bills.

Just theoretically asking, what would happen if I removed the external hold-off valve on my car? Would it allow equal parts pressure for the front and rear without the delay? It is leaking ever so slightly, just enough I see a drip on the inner fender.

 

OK, my wife is out of the halo brace now and I'm feeling itchy about finishing up the car since it is now 70 outside. I've got the rear end cover off because it was leaking and I'm really tempted to pull the trigger on some rear discs.

The 70 chassis manual (red book) says the little extra valve is the hold off valve and it goes to the front brakes. I'm thinking I can delete that if I put rear discs on and just use a separate proportioning valve for the rears. I believe the distribution/warning light block is literally just a block with the sensor, no check valves or anything necessary to change in a rear disc upgrade.

Harbor Freight has a double flare tool set so I'm thinking I'll get one and practice up for when this starts to burn a hole in my pocket.

Where would the residual pressure valve for the rear drums be to remove? Is it in the distribution block or is it in the master cylinder?