I would like to know if there is anyone who has a front disc/rear drum Camaro that was locking up the rear brakes.

And.. Did they cure the problem by fitting one of the proportioning valves that mount along the rear brake line, low down on the left-hand chassis rail.

I am trying to anticipate whether I will be needing one.

I have Delco-Moraine single piston calipers up front, an EIS 1-1/8" master, OE distribution/switch block, OE metering valve. There is a Ford 9" up back (:sad: ) using 13/16"dia wheel cylinders versus I think Camaro rears of 7/8"dia. (Only 1/16" smaller, but on a 1/2 larger dia drum, if that makes a difference.)
http://img78.imageshack.us/img78/5185/assembly1452ts.th.jpg (http://img78.imageshack.us/my.php?image=assembly1452ts.jpg) (It's back to front. The metering valve is there!)

The car used to have a 60/40 weight distribution (BBC alum heads) but will now have A/C as well.
Thanks.:)

PS. David Pozzi if you're reading, there used to be a link on your old website regarding the proportioning valve, but there doesn't seem to be on your new site.. Do you have any info on it? Thanks.

If you do not have a proportioning valve ( I recommend an adjustable one) with a drum to disc brake conversion you will need one. Disc brakes require more pressure than drums to operate. If you have enough line pressure to use your front discs (65% of your braking occurs here) then you will lock up the rear drums.

Further a disc brake requires a residual pressure valve (never releases all of the brake line pressure) to hold the pads loosely to the rotors. This is because the disc brake caliper pistons are huge by volume compared to the drum brake wheel cylinder; so the amount of travel must be limited (the job of the residual valve) to keep from emptying the master cylinder on every stroke.

Standard factory disc drums had two separate valves initially but they were quickly combined into one part by the early seventies. So finding the needed parts depends upon the donor vehicle you took the brakes off of (combination of caliper piston and master cylinder).

SSBC can find the correct parts if you have the information as to what size or origin of discs and master cylinder used. I recommend them only because they have helped me (excellent customer service with knowledgeable techs) in the past find obscure parts for my car.


Larger Dave

Thanks Dave. Is it possible that the master has a residual valve built into it? I think I read somewhere that some do.

Also, that's what I thought a metering valve's job was. To hold some of the rear brake pressure. Otherwise, what is it's job?

A metering valve is the proportioning valve. It is just an orifice valve (or needle and seat with an adjustable valve) that reduces the line pressure by means of a velocity drop (Bernoulli) going through a small hole into a bigger channel.


Larger Dave

Further a disc brake requires a residual pressure valve (never releases all of the brake line pressure) to hold the pads loosely to the rotors. This is because the disc brake caliper pistons are huge by volume compared to the drum brake wheel cylinder; so the amount of travel must be limited (the job of the residual valve) to keep from emptying the master cylinder on every stroke.


Why would a disc brake require a residual valve? When you release the pressure on the piston, the caliper relaxes. There are no springs to pull it back. On the other hand, drum brakes will completely recede from the drum unless you keep a small residual pressure on them. The standard OEM combination valve has a residual valve on the rear brakes as well as a metering valve on the front brakes, which keeps the front brakes from doing all the work on light braking.

Discs only require residual pressure if the master is below the calipers
or
If you are using low drag calipers without the proper QTU or high volume master cylinder.

Conventional caliper seals allow the pads to drag loosely on the rotors, no matter the volume.
Wheel cylinders may need a residual valve. Many systems have none. The purpose (besides if the master is below the wheel cylinders) is to keep the cup seals from sucking air when you let off the brake. The return springs inside the master cylinder return the pistons much more quickly than the wheel cyliders can return, resulting in a possible vacuum on the cup seals. These delicate cup seals also give reason for some books to recommend bleeding drum brakes after replacing only shoes. The cup seals can suck air when moved around without pressure on them. They are not unlike the seals inside a master cylinder which allow fluid to flow by them in one direction (upon release) and not the other.

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Regarding metering vs. proportioning... it can be said , and not altogether improperly, that the names are interchangeable, as metering suggests fluid flow control, and proportioning suggests pressure control, which is exising in both types of valve... BUT
in brake definitions the valves are NOT interchangeable.

A brake metering valve is for front brakes only, normally on a disc/drum system. It cuts off fluid flow completely to the front brakes under light pressure to allow the rear shoes to overcome their return springs. This results in near simultaneous braking from front and rear. The point at which it again allows flow can vary between models.

A brake proportioning valve is for rear brakes only. It begins reducing rear pressure IN PROPORTION to the fronts after a specified pressure has been reached. Below this specified pressure, it allows full flow. Above it, it reduces by a percentage. Both the percentage and the split point are different for different cars.
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As far as whether you need either of these valves or not, I couldn't tell you for sure. It is better to be safe than sorry, and an adjustable rear valve can always be opened to allow full pressure if you don't need it.

I do suggest that if you plan on street driving, rather than always on the track, I recommend larger rear cylinders necessitating the use of a rear proportioning valve (preferrably adjustable).
My reasoning for this is that during the light to medium braking such as you will see on the street, it's nice to have strong rear brakes. It stops easier and takes some of the stress off the fronts. During hard braking the rear valve will reduce pressure.
If you plan on driving in a lot of traffic jams, I would also suggest front metering. This keeps the front brakes from doing ALL of the work on continual slow/light stops. (For example some taxi&police vehicles have a metering valve which delays flow until a higher pressure is reached than their counterparts).


Oh, Mr. Pozzi's link you were looking for can be found here:
http://www.pozziracing.com/brakes.htm#Metering%20valve
http://www.pozziracing.com/prop_valve_info.htm
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additional notes from me
2ndgens increased the size of their rear cylinders in 1976 from 7/8 to 15/16, presumably to make better use of a rear proportioning valve.

80's Gbody owners have reported excellent results going from 3/4 to 7/8 cylinders.

Some mid 70s A bodys used 11" rear drums with 15/16 rear cylinders, and full-size front delco calipers.

Some Dodge Ram owners report good results going from 7/8 to 1 1/8 rear cylinders. Some eliminate their rear valve. There was a Dodge TSB on eliminating the rear load sensing valve in some situations on some trucks.

Thanks very much for your help and clearing up a couple of things.
Also, thanks for posting David's links that I was unable to find. :thumbsup:

For originality, I wish to use the OE proportioning valve. I wonder if I can fit it and "tune" the rears with different wheel cylinder diameters. Ummm :rolleyes:

Sounds like a plan, though might be difficult to implement, or might take a while. If you use the OE valve, I would guess that something between the current 13/16 to 15/16 would work fine with the 10" drums. If you can find some 7/8, just stick those in there. The extra 1/2" diameter in the drum shouldn't make much difference. It will depend alot on the rest of the car too.

I've been thinking of ways to adjust the OE valve too. I'll have to take my valve apart and get a good look at it.
Unfortunately if you go too strong on the rear, they could lock up at a bad time.

I would add a proportioning valve allmost allways uses a spring pushing against the valve, this spring force determines the pressures that reach the rear brakes. A proportioning valve isn't a simple needle valve.

http://www.stoptech.com/tech_info/wp_proportioning_valves.shtml