4 wheel disc breaks - rear brake drag - Master Cylinder and proportioning valve question

[Paulrfoster]

Hello all,

I have a '68 Cougar xr7 that came stock with front disc and rear drum. I converted the brakes using the Wildwood dynalite kit for front and back, all disc. I have an aftermarket cam, so I also installed a vacuum pump dedicated to the brakes. I used the stock booster and master cylinder.

I recently experienced some drag in my rear brakes after some stop and go traffic. I haven’t experienced this before, but I haven’t put on many miles since I did my brake conversion. My current master cylinder doesn’t seem to be the greatest quality and leaks at the top seal, so I recently bought this one:

Master Cylinder - 4 Wheel Disc - Power Brakes - New ~ 1967 - 1973 Mercury...

I believe the issue is that my residual pressure valve on the stock master (front disc / rear drum) is likely 10 PSI for the drum. The new one is specifically listed for “4 wheel disc brakes” but I wanted to confirm this one has 2 PSI for both the front and back. Anyone have any thoughts / feedback?

Also, I have the stock distribution and portioning valve combination under the master on the drivers side. My brakes have felt okay, but I am considering an adjustable proportioning valve. Any recommendations on a replacement that fits exactly where the stock one is so I do not have to redo any lines?

Thanks

[Royce]

The stock one is shown here. Cylindrical part in the middle.

[Paulrfoster]

Thank you for the photo.

That is what I have installed, just curious if anyone has a an after market / adjustable one that fits where the current one is. Prefer not to RE-run new lines if I don’t have to.

[Royce]

The common aftermarket one made for Summit Racing has a knob. Most folks plumb it so the knob can be adjusted while driving which is pretty easy using line assemblies from any auto part store.

[yearby]

You don’t use/need a residual pressure valve on disc brakes. It is only used on drum brake assemblies and it is to maintain a small pressure (about 2 psi) on the wheel cylinder rubber cups so they don’t suck air/moisture in when the brake pedal is released. Some Ford don’t even have a residual pressure valve, just springs inside the wheel cylinder.

On a disc brake assemble, the Caliper rubber seals return to their natural position and with draw the pistons and thus the shoes come off the discs. So if there is residual pressure on the caliper pistons, the pads will not withdraw and thus drag.

The Cylindrical part as noted in Royces picture is the residual pressure valve, you need to remove it completely.

The other part in Ryces picture is the combination block , it has the brake warning switch which will light the brake light on the dash if one of the brake circuits fails, you still need to us it.

I am not sure if the 68 Cougar has a preset proportioning valve in the combination block, I don’t think so.
“You will need to check, I can’t remember at this moment.”

If it has, it will need to be disconnected or by passed some how, as you will need an adjustable proportioning valve. The will be no proportioning valve in the master cylinder or the brake warning switch will not work. Put the adjustable proportioning valve in the rear brake line after the brake warning switch.

When you adjusting the brake proportioning valve, make sure the front brakes lock up first.

Extreme Warning here. If your rear brakes lock up first at high speed, the vehicle will go into an uncontrollable fishtail side, where the rear of the vehicle will try to pass the front of the car, the result is the car will flip.

Just an update on Royces picture.

As an added bonus, you may want to look into.-

There is a regulated pressure switch that goes on the front brake line of disc brake cars, it was brought out by Chrysler in the 70’s. I have forgotten what it is called. It delays pressure to the front discs just long enough to have the rear brakes come on first. The result is the car will stabilize in emergency braking.

Hope this helps
Peter

[Royce]

Peter is confused. My picture shows the 1968 - 69 proportioning valve.

[Paulrfoster]

Thanks all for the info.

As a point of clarity, on the ‘68 my understanding is the following:

1 - the master cylinder has a residual valve inside it behind the port where the brake line goes in. It has a 2 psi valve did disc and a 10 psi valve for drum.

2 - the combination / distribution block is only to distribute the fluid and activate the brake light switch. I have taken this apart and there is a cam inside that slams to one side if the front or rear lines are open setting the light and limiting fluid to the impacted side.

3 - the proportioning valve is the bullet type cylinder in the above photo. It only balances flows between front and back. I wanted to replace this with an adjustable one.

Side note, I believe this valve was built in to the distribution block in ‘70+ cougars.

I will replace the m/c wirh the one referenced above and report back.

[yearby]

Royce, do you have a picture of the 1968 - 69 proportioning valve as illustrated in your 68-69 workshop manual that you can show me.

I don’t have any cougar workshop manuals.

So I can try and work out where the confusion is?
Could be we have different terminology for each component.

Thanks Mate

[gah]

The distribution block is the square machined block that distributes hydraulic fluid from the MC to the front and the rear brakes, the proportioning valve ( cylindrical component ) below and to the right of the block. It is part of the rear brakes, and has nothing to do with the front disc brake fluid circuit. It’s purpose is to regulate / ‘cushion’ the flow of brake fluid to the rear brakes, to prevent a panic lock up when one isn’t exerting maximum pressure on the brake pedal.
Here’s an image of the two components ( from WCCC ) :

[Paulrfoster]

Great pic, thanks for sharing. Exactly what I have. If I replace the proportioning valve I will find one with 7/16" fittings.

Once I get the master cylinder in that I noted above, I will share results on brake drag issue. It only happened once during bumper to bumper traffic.

Thanks!

[Royce]

This is the 1968 Mercury Cougar Shop manual illustration. I see no confusion here.

[yearby]

Royce, yes mate, all is good, my mistake, good pick up.

[yearby]

If getting new proportioning valve, check, I think adjustable proportioning valves for drum brakes and disc brakes are different. Due to the higher pressure discs work at.
???

[Royce]

They are the same, because the braking required on front disc / drum is nearly identical to that required by rear disc brakes. The only advantage to having rear disc brakes is the resistance to fading over multiple stops. Drum brakes work really well on the rear, because most of the braking is done by the front wheels. It’s a case of weight transfer and adhesion.

[PonyCarMan]

@Royce

It’s a case of weight transfer and adhesion.

You sound like a foundation brake engineer. I never hear anyone refer to adhesion!

[DieselD]

I simply replaced the prop valve with an adjustable wilwood for my aftermarket disc drum setup. Works well and only took bending up two lines to get it to fit in nicely.

[TMACK]

Can you please post the model number of the wildwood one you purchased?

[DieselD]

I believe this was will work for you. Appears they have been updated over the years. I was on the way to a cars and coffee event today and had to make a panic stop, the willwoods did their job and didn’t yank me all over the road.

Wilwood Disc Brakes - MasterCylinder No: 260-8419

Here is mine