Disc brake conversion

[BWhitmore]

Need some advice concerning the conversion to front disc brakes and a dual master cylinder.  I have installed the "stop/drop" disc brake system on the front of my 57 ranchero, stock drums at rear,  with a dual master cylinder from a 67 Mustang (disc brake).  The large reservoir in the master cylinder is hooked to the disc brakes and the small reservoir hooked to the rear brakes.  The two lines coming from the master cylinder are attached to an adjustable prop valve mounted on the frame.  The prop valve is adjustable and also includes a port for the brake light switch. 

While bleeding the brakes I continue to get a soft pedal and need to continually pump the pedal to get any feel.  I checked the master cylinder ports to see if there is a residual check valve in the rear drum brake port and there is.

Do I need to install a 10 lb "in-line" residual check valve for the rear drum brakes?  If yes, should I remove the residual check valve in the master cylinder?  Do I also need to install a combination valve (combination metering and prop valve) instead of the adjustable prop valve?

Any help would be appreciated.  Thanks.  Bill Whitmore

[Ford Blue blood]

Bill you do not need the "residual pressure valve" as the master is above the wheels.  Gravity keeps the wheel cylinders from draining back into the master, a common problem on street rods that have the master under the floor.  I would install the combination valve and remove the adjustable proportioning valve.  Only put the adjustable in if you experiance rear wheel lock up before the fronts lock.  Make sure your rear brakes are adjusted out so that they just begin to drag on the drum.  Pump the brakes and repeat the adjustment.

After you put the system together again try my method of bleeding.  Open the bleeb valves on the wheel cylinder/caliper farthest away from the master and relax.  Gravity will fill the lines and push any air out.  When it starts to drip, close it and open the other side, it will drip sooner.  Repeat for the other end.  I have had great success doing my brakes this way and have never had to go back to redo

[Frankenstein57]

Bill, I thought that was known as the two beer method?  My first attempt at a system like that, my brakes would come in hot, dragging. The unit I used had a different booster on it because of shipping damage. Also an adjustable prop valve, I got tired of screwing around with it so I installed a different master, with a prop valve from CPP. I had that on the shelf for my 57, brakes have been perfect ever since.   Mark

[KULTULZ]

I am going to open a can of worms here... 

All drum brakes need a #10 residual valve regardless of MC positioning (it was included in the OEM MUST DISC/DRUM MC). The valve keeps a light pressure on the wheel cylinders keeping the shoes slightly extended to contact the drum more quickly. It also keeps the wheel cylinder cups from briefly collapsing after brake release allowing air to be ingested past them.

A combination valve (METERING - PRESSURE DIFFERENTIAL - PROPORTIONING) has to be included. An adjustable proportioning valve, unless properly calibrated for the car (gauges and/or skid pad) will get you into trouble on a slick road surface.

The most common one offered by CHEV Brake Kit vendors is from a Chevelle which should give the correct brake bias.

Proper pedal height cannot be achieved without this valve.

This is only offered info and your mileage may vary. Please be nice.

[Ford Blue blood]

All drum brakes need a #10 residual valve regardless of MC positioning (it was included in the OEM MUST DISC/DRUM MC). The valve keeps a light pressure on the wheel cylinders keeping the shoes slightly extended to contact the drum more quickly. It also keeps the wheel cylinder cups from briefly collapsing after brake release allowing air to be ingested past them.

The master cylinder he is using has the residual in it already.  A properly set up master cylinder is fundamentaly a straight shot from the resevore to the wheel cylinder/caliper.  That is to say, if the line ports were located under the master instead of the side you could see daylight through the resevore, through the piston at rest and back to day light (providing it is a disc/disc, disc/drum will have the residual in it).  The purpose of the external-inline residual valves we all read about in various car books is for the master lower then the caliper/drum position.  There are no "in line" residuals in the millions of production autos out there running drums (as stated earlier, the residual is in the master itself) and none in the many thousands of disc/disc autos out there, providing both types are using fire wall mounted masters.

[KULTULZ]

The needed residual valve is indeed in his MC as it is an OEM type (his being Disc/Drum). #2 and #10 inline residual valves are readily available.

Most of these mongrel GM disc changeover kits come with a GM Disc/Disc MC and have no residual valve(s). This where you would use the inline #10 on the drum brake circuit.

The #2 is used when the caliper/wheel cylinder is mounted above the MC reservoir level to prevent brake fluid drain back.

[rmk57]

I used a Granada disc/drum master and had to use a #12 residual check valve to firm up the pedal as mine still has drums in the front. It does help somewhat.

[Ford Blue blood]

That was because the front section was for disc and had no residual valve in it as the rear section did.