Technical Front disks stop car before rear drums..

Hi folks. I'm finding that my 52 seems to stop primarily with the front disk brakes. The car is set up with fairly typical stuff. 1" corvette style mc. 3/4" wheel cylinders for the rear drums, front disks, 10lb residual valve for rear, mc is mounted on firewall with Walton fabrication kit and has a power booster. I believe I've adjusted the shoes properly as well. With the rear end in the air and the car in drive, I have to really step on the pedal in order to get the tires to stop spinning. Also, while the rear is in the end, the pedal travels about an inch with seemingly no affect on the rear drums. I can tell you that when the car is on all fours and driving, I do not have to step that hard on the pedal for it to stop, meaning the front brakes always slow the car before the rear drums do. While road testing and slamming the brakes, At this point all wheels stop because I've slammed the pedal. Is this normal? Should the front brakes do the majority of the stopping and only in harder braking should the rears kick in? I've bled the system with Motive pressure bleeder and various other methods with same results. Any input would be much appreciated.

 

does the car have a proportioning valve?

 

It has a adjustable valve going to the rear drums. Currently, its fully open.

 

On a production car the engineers spent much time developing the system. As hot rodders we take parts and pieces and engineer our own system. You may need to balance the system with an adjustable proportioning valve. This will allow you to change how much pressure goes to front brakes versus rear. Front brakes do most of the work. That is normal. But with less than ideal traction like in rain you want the front rear balanced so you don't do donuts as soon as you touch your brakes. On race cars we use them for adjusting bias as track changes or tire grip goes away.

 

The front brakes do the majority of braking in any case. If the car stops straight and doesn't try to swap ends under hard braking, it's good.

 

Do you need the 10 valve with the master on the wall ?

 

Probably not... but I supposed it didn't hurt? not sure... I was there from when the mc/ booster was under the cab before I moved it to the firewall. I'd just have to replace it with a union.

 

Id try it ,if the master is for disc drum its not needed. Other than that the adjust on the rear is a tad loose. I always do the rears a little tight to break in the shoe to the drum.

 

You still might need the res valve, you need to check your master first before you take out the valve. Which way did you turn the adjustable prop valve, all the way in or all the way out?. Lastly if you need more pressure you might need to up size the rear cylinders.

 

I have GM front discs in my '59 Ford, stock drum brakes in the rear. Master is a 1" Mustang/ Fairlane disc-drum unit.

I do have a SSBC proportioning valve with rear bias adjustment, which is wide open. I run no residual valves and the car stops straight and true. I've done a couple of unintentional hard stops and nothing locked up. I usually get into arguments about the no residual valve thing, but its hard to argue against success. I would ditch the residuals and see what happens.

 

The front brakes do about 85% of your stopping. I have noticed some cars with all factory installed brake parts act like what you describe. 70's GM intermediates come to mind. I am not saying that it is correct for your application.

 

Your post say's "corvette style" MC. Is it a corvette style or a true corvette MC. I've always used 70's and 80's disc/drum MC with success. The proportioning valve is the right move but with a firewall mounted MC I've never used a residual check valve. The proper MC already has one in it.

Pat

 

As already stated, the front brakes should always do most of the braking. What is your '52, and are the rear brakes original?
Adjustable prop valves are normally "fully open" or maxed out when adjusted fully CW, but follow the arrow on the knob to be sure.
Drum brake residuals are really needed on older pre-'70s brakes, regardless of master location, and will only help on newer drums, not hurt, by slightly minimizing pedal travel.

 

Are your lines plumbed front port to disc's and rear port to drum's @ the master ?

 

If you are using one of those br*** combination valves located adjacent to the M.C. then that is how it works. The front brake circuit is delayed thru a 'metering' valve, the rear brake circuit apply's first and the pressure restricted to about 400psi. The problem is that the front discs would see 1000+psi and react quicker than the rear drum, the car would nose dive upsetting the brake balance (bias) shifting weight to the front adding to the front brake and unloading the rear.
M.C.'s prior tot the development of the combination valve had internal circuits, like 1934coupe referred to.

 

Proportioning valves and residual valves aren't enough for a disc/drum setup. If a master cylinder doesn't provide for this, a lot of factory applications use a "combination" valve which combines a metering valve with a proportioning valve. The drum brakes have slack in them so they need do get fed first, but then they only do about 30% of the overall braking. The combo valve feeds them first, but send them less pressure overall.

 

Just remember front disc take 2 pounds back drums take 12 pounds of pressure so u need to adjust ur proportion valve accordingly that's the way I try to set mine up after a lot of research, jmo HRH


Sent from my iPad using The H.A.M.B. mobile app HotRod Harry

 

Just some clarification.
bob52 stated using an adjustable prop valve, and with too much front braking as the problem.
Combination valves came in 3 basic styles: fluid distribution and a (+/- 400 psi) pressure differential switch; distribution/press. diff. and metering; and finally distribution/press. diff./metering. and fixed value proportioning. Combo valves were common with all dual systems from '67 on, and probably with Cadillac's dual system in the early '60s.
Metering holds off the front discs around 100 psi, or about the pressure needed to overcome the drum shoe return springs. The purpose was to have both axle's brakes apply at about the same time, and had more to do with pedal "feel" along with vehicle response, depending on ch***is dynamics. Metering was used on and off throughout the years but not on all disc/drum systems.
Any "internal circuits" would be drum brake residuals, and while found on most disc/drum masters up to the mid '70s, may not be installed on some aftermarket or replacement masters, and is something that should be checked with the master you intend to use.

 

Pat, it's not an actual corvette MC (I think I've hear them called that sometime and that's why I referred to it as such, lol), it's this one here...
http://www.speedwaymotors.com/Aluminum-Master-Cylinder-w-Stainless-Bore,50598.html

It's a 52 Chevy Styleline with non-original rear end. They stuck a rear end out of a early 80's camaro. For the prop valve, it's fully open per the arrow on the valve. Thanks.

Hi Johnny. Yes, the lines are correct; front to disks, rear to drums. Thanks.

 

I actually had a br*** combo valve as well but with the same affect.

 

Thanks for the detailed info! Currently, I don't have the combo valve installed. When I was running it, it had had the same affect. Also, the MC I'm using does not have built in residual valves.

 

So, what I'm gathering is that this might be OK behavior? I put my brother's K5 rear end in the air and tried the brakes for comparison and it felt a lot better than my car. Somebody mentioned bigger wheel cylinders. If I do this, what will be the affect? Will the rear drums engage sooner than with the smaller wheel cylinders I'm running now? If so, I'm thinking I could then use the prop valve to adjust for any rear brake bias (currently, it's fully open to allow normal pressure to the rear).

 

Proportioning valve OP says is full open..If by full open as in water faucet; full CCW he has min rear pressure to brakes...CW is full rear brake pressure..Starting point should be half way number of turns between Full CCW and full CW ..

 

If the prop valve is now maxed out with insufficient rear braking, the Camero wheel cylinders (or brakes) are probably too small. I would measure and find out if larger cylinders are available, and install.
The advantage of an adjustable prop valve is the ability to balance the system, so if larger cylinders result in too much rear braking, simply reducing the pressure setting on the valve will lower the knee or crack point of the valve. Usually, these valve's adjustment range is from about 200 psi to over 1200 psi before proportioning starts to work.
You want the rears to do their share of braking without sliding before the fronts, but high decel stops without any slide is best!

 

I think this is the next thing to try that makes sense to me. It has 3/4" bore cylinders but I can go up to 7/8". I could always adjust down the pressure like you say. Thanks for the input!

 

My bad, I forgot you knew the 3/4" bore. I would go at least 7/8". I had to increase from 13/16" to 1" on my '32 to get the 10" rears to work properly, primarily because of the large OD tires.

 

Back to basic's with this one sentence and let's forgot all the other stuff for just one moment. Have you verified that the primary circuit of your master is working ? That 1" of travel has asking thinking because if the primary circuit doesn't make pressure it will compress the primary piston spring until it begin's to move the secondary piston which is your front brakes.

 

Good point. Am I'm now very familiar with this diagram Yes, the front brakes stop very well and I don't see leaks.

 

So.... yesterday I ended up replacing the wheel cylinders with bigger bore ones. The only thing available for me were the 7/8". The style it's using are the ones with the clips in the back to hold the cylinder in place so there aren't many options as far as sizes. I went from 3/4" to 7/8". I bled and adjusted the rears and they seem to function better. For some reason, the p***enger wheel stops as I would expect but the driver side requires a little more on the pedal to see it come to a stop (again, rear end is in the air). I'll have to put the wheels back on and do a road tests to see how it feels. Thanks a lot for the input, fellas.