A Quick Question

[69Nova]

Ok so my tax return is in. So I am going to get some new front brakes for the car. My question is will it be ok if i just upgrade the front to a set of willwood's and leave the stock rear drums for now. The rear drums were completly rebuilt about 4 months ago. I dont think the brakes i want are too crazy they are going in a set of 16" wheels.

Thanks for he help.

[6'9"Witha69]

Are the fronts Disc already? If so then go ahead. If not, then the rear brake holdoff valve will not be there and getting the thing installed (retrofitted) can be a pain. Also Metering Blocks are different on Drum/Drum and Disc/Drum applications. Regardless, put a proprtioning valve in the rear line for adjustability.

[69Nova]

The whole car right now is non powered drum brakes. So if I just installed a proprtioning valve I could make it work. How would I go about tuning so that it would work corectly?

[dhardison]

Andrew, your factory manual brake metering block will work fine with front discs & rear drums but you'll have to use a proportioning valve to limit the rear brake pressure. You'll also have to change out your drum/drum master cylinder to a unit that'll support the discs.

As far as tuning... Get everything installed/bled properly, and start with the proportioning valve wide open. Go out for some 'test runs', but be CAREFUL until you get the prop valve adjusted properly. At first your rears will lock up very quickly, LONG before the front discs begin to work (i.e. you'll basically only have rear brakes at first). Adjust the proportioning valve until the rears stop locking or until the car stops like you prefer.

Are you going with a power brake set up or staying manual? My 4-wheel manual discs from Touring-Classics are awesome! I HIGHLY recommend them as you won't find better customer service and product backing. However their C4/C5 brake kits more than likely won't fit inside 16s.

Dan

[69Nova]

As far as tuning... Get everything installed/bled properly, and start with the proportioning valve wide open. Go out for some 'test runs', but be CAREFUL until you get the prop valve adjusted properly. At first your rears will lock up very quickly, LONG before the front discs begin to work (i.e. you'll basically only have rear brakes at first). Adjust the proportioning valve until the rears stop locking or until the car stops like you prefer.

Why will the rears lock up first. I would think the front disc brakes would lock up first?

Are you going with a power brake set up or staying manual? My 4-wheel manual discs from Touring-Classics are awesome! I HIGHLY recommend them as you won't find better customer service and product backing. However their C4/C5 brake kits more than likely won't fit inside 16s.

I am going to stay manual.

Again thanks for all the help Dan.

[dhardison]

Why will the rears lock up first. I would think the front disc brakes would lock up first?

Weight transfer will generally make the rears lock up first.

Dan

[69Nova]

Weight transfer will generally make the rears lock up first.

I forgot about that.

[6'9"Witha69]

That is part of it. There is a brake holdoff valve on Disc/Drum cars. It holds off line pressure to the rear brakes for a brief time to allow the fronts to engage first.
Taken from the inline tube website: "This valve was used to hold off pressure to the rear brakes so the front brakes would go on a split second before the rears. This was also considered a correction valve for the 1970 model year. During extreme braking conditions this valve prevented the car from getting sideways and evened out braking loads."

Secondly, Drums just lock. That is a big reason disc brake conversions are favored.

[69Nova]

Secondly, Drums just lock.

Oh I know all about that. A couple times in packed traffic I have ben forced to use evey bit of knowledge gained from GT4.

[6'9"Witha69]

Yeah, drums have been the reason for a few of mine and my friends cars to go the way of the wrecker. And not because we were jerkin around either.

[dhardison]

Yeah, drums have been the reason for a few of mine and my friends cars to go the way of the wrecker. And not because we were jerkin around either.

Yep rear drums are an accident waiting to happen. Here's what happened to a friend's Nova when the rear drums locked in an emergency stopping situation. The rear of the car jumped sideways and he slid in to a light pole.

Instant parts car...................... :-(

[Ralph LoGrasso]

Yep rear drums are an accident waiting to happen. Here's what happened to a friend's Nova when the rear drums locked in an emergency stopping situation. The rear of the car jumped sideways and he slid in to a light pole.

Instant parts car...................... :-(

I had a similar thing happen to a buddy of mine with a '70 Challenger. He had just gotten his car back after getting his tranny rebuilt and some other tidbits ($3k in work). Cruising around that evening someone cut him off, he got hard on the brakes to avoid rear-ending the guy, the rear drums locked and he went around 180*. The rear (wheel) of the car smashed into the a curb, luckily for him there was no body damage, but the rear end was shot as were both rear wheels and some other minor stuff. He wound up shelling out another $3,000 to fix / replace everything.

[69Nova]

Wow. Honestly I dont know what I would do if I wrecked my car like that. One time I had to get my car to stop the rears locked up and I had to pedal that car to keep it in the middle lane of three.

[CarlC]

The F/R brake bias is the primary factor causing rear brake lockup on aftermarket systems. This is especially true when stock rear drum brakes, with a low required line pressure, are mixed with an aftermarket front disk caliper that requires a significantly higher line pressure to work. The two do typically do not mix well.

For those that remember Craig Boone, he spun out twice during a 0-100-0 mph derby. His car had 4-wheel disks, but the rear calipers were poorly matched to the fronts, and the rear brakes locked prematurely. He spun out, and the requisite changing of the shorts was needed.

[BonzoHansen]

The F/R brake bias is the primary factor causing rear brake lockup on aftermarket systems. This is especially true when stock rear drum brakes, with a low required line pressure, are mixed with an aftermarket front disk caliper that requires a significantly higher line pressure to work. The two do typically do not mix well.

In a situation like that, would an adjustable prop valve help to dial that in? I ask because I can’t ever remember seeing one on a disc/drum setup.

[HILROD]

You have to put at least a factory disc/drum valve in your car. The drum/drum valve will not work. Ask me how I know! You can get a GM valve or a repro one for a newer G-body or S10 for a little less rear bias. I have done 3 or 4 swaps like this and they all worked great. If need be you could use an adjustable rear after to fine tune it. I think the factory drum valve will allow the calipers to retract too much. I do now for a fact that is a scary unsafe ride done that way. FRANK (I just edited this. I do have a stock disc setup if that could help you ?)