Porportioning valve

RobG

RobG

Member
Ok, on my 68 Scout 800 with the 152 engine I have almost worn out the rear brakes after only 500 miles. The brake pedal is very hard to push and in order to stop I have to push extremely hard. Also, I'm pretty sure the rear brakes are not releasing which is why they are worn out. We tore them apart today and found nothing wrong. The emergency brake is releasing so that is not the problem. The master cylinder has been rebuilt and tested and the front brakes look fine. I'm wondering if the porportioning valve is bad? Suggestions please.

Rob gimmel
 
Or could it be the master cylinder? Is there a relief valve somewhere that might be sticking?

Rob gimmel
 
On my 79 with a Dana 44 there is a check valve that holds about 10 psi at all times. To put the drum back together did you have a hard time even after releasing the pressure from the cylender. It May be possible that the check is holding more pressure than designed which keeps the pads engaged on the drum assembley. Just a thought. Can you spin the tires freely with no p-brake engaged?
 
Where is the check valve located? Is it in the master cylinder or elsewhere?

Rob gimmel
 
Rob: in a drum/drum brake system, there is no proportioning valve.

There are residual pressure check valves in the master cylinder under the flare connections that the brake lines connect to. They would be the big nuts on the side the inverted flare connections go to for the rear and front brakes. Under the nuts there are rubber umbrella looking things that hold approximately 5 psi of residual pressure to keep the drum cylinders on the ready so the pedal doesn't have to be stroked so much to build braking pressure.

Hard pedal pressure could be hydraulics such as master cylinder bore vs. The wheel cylinder bore I.d. 1968 and 69 800's used a step bore master cylinder 1" x 7/8". The wheel cylinder bores where 1" front x 7/8" rear. The other possiblility is pedal geometry, though I doubt that. I have seen the wrong master cylinders installed, and the front and rear wheel cylinders swapped leading to hard pedal pressure.

When the brakes are rebuilt, the backing plate contact points for the shoes have to be clean and free of burrs and as much as possible free of grooves. The new shoes can hang up on these old wear points and not retract fully leading to excessive shoe wear.

If the front and rear axle brakes are not adjusted in sync with one another, one axle could be doing all the braking leading to hard pedal effort and premature wear. Adjust the brakes with a slight drag on each drum and back off 2 to 5 strokes of the adjusting spoon for each. Take the rig to a soft road surface, like a gravel one, and at 30 mph apply the brakes hard, whatever wheel/s lock up are too tight or the others too loose. You want a balanced braking with no pull and no wheels locking up over the others.
 
I knew someone with more info would chime up. The drum drum scenario on the Scout I is different than the disc drum setup on the Scout but all would have the check valve. It's good to know that the check valve is good for 5 psi as opposed to 10 on a Scout II application. You wouldn't happen to be close to houston would you. I could use some expert help with my system just to be sure it is working at optimum.
 
it's good to know that the check valve
Click to expand...

Thanks for bringing that up. I went and checked the manuals, and you are closer. Early single circuit systems the manual sez about 6 to 8 psi; and dual circuit systems in the later manual it shows about 6 to 18 psi. The main point is there is pressure to keep the cups flexed and the drum brakes at the ready.
 
The service book shows a proportioning valve in the system. I'm beginning to believe I have a problem with the master cylinder. Is there a good diagram that shows how the cups go on the pistons and how the relief valves are installed?

Rob gimmel
 
I am assuming you have stock brakes on your 1968 800. What I forgot to ask is what is your brake setup? Is it stock, or have discs been swapped on such as the front axle?


The service manual is usually a compilation of sections of the factory service manuals. On hydraulic brakes there is a general service section that covers master cylinders, and accessories such as the boosters and combo valves. The vechicle specific sections cover the actual brakes be it disc or drum and then based on size be it 9", 10", or 11". Put together for a "service manual" for us, we'll see different options but not necessarily what is on our rig. Many folks also confuse the brake warning light switch with being a proportioning valve or combo valve. If your rig is a drum/drum set up, most likely what you have is the brake warning light switch.

The manual should have the diagrams for the parts arrangement.
Most master cylinder kits have the pistons pre-assembled, though some have just the cups for the secondary pistons as the primary pistons are discarded and you get a new one.

For a step bore master, the residual valves order are from inside out: spring, disc with bevel side facing outward, rubber, gasket for the big nut and then the nut.

For a straight bore master: spring, rubber, and then the inverted flare insert.
 
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I do have stock brakes on my Scout. The service manual does show the master cylinder but is not real clear on which direction the cups go and how the relief valves go in. I might have a source for an old master cylinder that I can take apart and see how it's put together.

Rob gimmel
 
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