Brake Combination Valve

If you're using a caliper and single disc like a Karmann Ghia, stock beetle drums in the back and the same size wheels and tires all the way around you'll get differing opinions as to whether you need residual valves or not, and I've never heard of using different ones at both ends at the same time. With the right master cylinder I've seen this set-up work just fine without any valves. 

 

On a beetle I have run Ghia discs with smaller tires up front (I've tried both 135/15's and 145/15's) with beetle brakes and taller tires (185/70's) on the back and the front brakes would lock up way too early, even on dry pavement. Wet or dusty pavement and you had to be very careful. Type 3 rear brake/drum assemblies in the back balanced the combo. Rear discs would do the same.

I agree.

 

I'm running the same brake configuration as in your picture.  The Master Cylinder is from a super beetle sedan with disk front/drum rear - bolt right in.  

 

I have no residual valves in the front circuits, just straight tubing to each caliper.  The line running to the rear has a single 11 pound residual valve in it to hold the shoes a bit closer to the drums when at rest.  I ran for years without that rear residual valve and installing it made only marginal difference.

 

All seems to work OK.

Late model Brazilian Beetle (Fusca) and Mexican Beetle use a dual circuit single master cylinder that is already biased for front disc and rear drums and do not need any residual valves.

 

That said, your layout is proper for adding residual pressure valves.

Thanks gents this is excellent feedback.

Just a matter of interest what is an ideal balance between the front and rear brakes on a speedster with disc/drum setup?

 

What does the combination valve do? Does it adjust F/R brake bias?

This is the info that I have on combination valves.

 

The combination valve contains a proportioning valve, metering valve, and a pressure differential switch all in one.

 

Metering Valve: The metering valve is in charge of delaying the application of the front brakes just long enough to overcome the rear brake springs. It is used with front disc rear drum braking systems. When the brakes are first applied pressure quickly overcomes a spring in the valve and closes a port to the front brakes temporarily reducing fluid flow and pressure. When the pressure builds to a predetermined amount (75-100psi) flow and pressure to the front calipers are returned to normal. When the metering valve fails the vehicles front brakes apply early and the vehicle tends to nose dive.

 

Proportioning Valve: The proportioning valve is used to prevent rear wheel lock-up. These valves achieve brake balance by reducing fluid flow and pressure to the rear brakes during hard and sudden stops. This valve helps achieve better braking only during hard and panicked stops. Most of the time, during light and moderate braking the valve lies dormant. A faulty valve would tend to lock up the rear brakes because the vehicles inertia transfers the weight from the rear to the front while braking.

 

Pressure Differential Switch: The pressure differential switch is used to inform the driver if there is a leak in the system resulting in a loss of pressure. It is used in dual master cylinders to detect a difference in pressure between the two sides. If there is a difference in pressure it will complete a circuit and illuminate the red warning indicator lamp.

 

 

Residuals valves is not part of the combination valve rather residual valves are used to maintain constant pressure on the brake system.  For drum brakes, a 10 lb. residual valve is used to hold pressure against the springs in the drum.  This allows the drum brake to engage more quickly and reduces pedal travel.  For an under-the-floor master cylinder brake system, a 2 lb. residual valve is used for disc brakes to keep brake fluid from flowing back down into the master cylinder.

 

 

 

 

you're getting too technical.  just run a line from the master cylinder to a T then a line to each front caliper.
I would not run the residual valves initially and try it that way. It really depends on if your master cylinder has a built in residual valve for the frt brakes. If your looking to race the car then you may want to try a proportioning valve first or do it right and have a pedal assembly with master cylinders for frt and rear with a balance bar adjustment.
Otherwise keep it simple and you will have less headaches

I agree with Anthony....keep it simple, and if you find your rear brakes locking before your fronts you can always install a proportioning valve  later on (I did this with my track Miata).