Brakes

I need a few ideas on my brakes. I have a Heidts front with the GM brakes. I believe their 79 Malibu stuff. I got the braded brake lines. On the the rear I have a 9 inch ford with drum brakes. I have the rear flex line in rubber. I have an 8 inch GM booster with a disk/drum master cylinder. I have a combination valve (I believe it's called) from a mid 80's Disk/Drum GM car. I have an adjustable proportioning valve and a line lock.

1. Are the flare fittings for ford and chevy the same and what size are they?
2. Should I use the combination valve?
3. Is the wire on the combination valve for brake lights? Should I use it or the switch behind the pedal?
4. Any ideas on how to hook this stuff up?
5. Later on, I believe I'm going to buy the Explorer Disk Brake Setup for my rear end. What all will I have to change?

I was thinking of mounting the combination valve, adjustable prop. valve, and line lock down on the frame to keep the engine bay clean. I don't have a flare tool. I was going to buy some different lengths of brake line and bend it up. The only problem is getting the stuff the right length. I did this on the trans lines and it worked ok. I needed to shorten one line some and I did this with ferrels. Can you do this on brake lines? Thanks

Hi siderman,

1. Most domestic vehicles (such as GM & Ford) use brake lines with a standard 45 deg inverted flare and standard US threaded nuts. I have seen some that use lines with "bubble" flares though. The most common brake line size is 3/16" but a few are 1/4". The standard size flare nut for 3/16" lines has 3/8"-24 threads but some applications (typically master cylinders and combo valves) use oversize flare nuts. Inverted flare adapters are available in a variety of sizes if you don't want to cut the line, swap the nut, and reflare. The oversizes vary so it's probably best to take your master cylinder, combo valve, and such right to the store with you so you can find the proper adapters.

2. Yes, I would recommend using the combo valve. Most GM combo valves include a metering valve that slightly delays the application of the front discs. That helps reduce pad wear and improves stability by letting the rear brakes apply first.

3. The electrical connector on the combo valve is designed to activate a brake warning light on the dash if the front or rear half of the brake system looses pressure due to a leak. It's not used to activate the car's brake lights. Use the switch at the brake pedal for that.

4. I'd recommend following the basic layout that GM used in the late 70's and early 80's. Run a line from the MC front outlet to the combo valve front lnlet. Run another line from the MC rear outlet to the combo valve rear inlet. These lines can be fairly long if you want to hide the combo valve down along the frame. The combo valve has 2 front outlets. Normally, you'd run a line from each front outlet to each caliper flex line. But with a line lock, you can plug one of the combo valve front outlets. Run a line from the other front outlet to the line lock. Then put a Tee fitting after the line lock to split off to the left & right calipers. For the rear, run a single line from the combo valve rear outlet back to the flex line. The combo valve already contains a proportioning valve but you can add the adjustable one into this line if needed. On the rear axle, put a Tee fitting at the end of the flex line to split off to the left and right wheel cylinders.

The lengths of pre-flared line are definitely the way to go if you don't have a flaring tool. As you mentioned, they don't always come in exactly the lengths you need. Sometimes a bit of "creative routing" of the line (adding a few zig zag bends or loops) can be used to take up some extra length ... doesn't always look that neat though. And sometimes you can join two shorter lengths together (using an inverted flare union) to make a custom length. Say you needed a 32" line for example. Nearest stock sizes are 30" and 40" but you could make the 32 by joining a 12 and a 20. Of course this still won't work for all lengths but it helps in some cases.

Compression fittings are fine for lower pressure applications (like your trans cooler lines) but I wouldn't recommend them for brake lines. I think you'd be better off buying, borrowing, or renting a double flaring tool if you end up needing line lengths that can't be put together using the above listed methods. Or cut the lines to length and take them to a shop that has a double flaring tool.

thanks for the great write up Ray. Really really helps. I'm going tomorrow and pick up some tubing. I had a small piece of old brakeline that fit both the front and rear brakes. So I know what to get. I'll take the rest with me like you said.

One question though. I've had the combination valve for a while. I lost the electrical connectors for it so I'm going to have to make them. It looks like their is a connector on the front and top. Is this so. Is it normally open or closed when pressure is applied? Is this how it works or does one side go to ground when pressure is applied or relieved. The valves I looked at aftermarket only have one connector it looks like.

I looked at a diagram on autozones site and they talked about the electrical connector, as in one of them. they called the thing on the front a center pipe? Wonder how much new combo valves cost?

One other thing. If later I upgrade to Disk/Disk, will I have to run another brake line to the back, or do they split at the rear like a Disk/Drum setup. Thanks.

Hi siderman,

You're welcome!

The combo valves I'm familiar with only have one electrical connection (a round pin) located in between the front and rear inlet ports. The metering valve has a pin that sticks out through the front of the combo valve. On some valves it's covered with a rubber cap. On others, the tip is exposed through a hole in the rubber cap. Some bleeding procedures require that pin to be held in. In any case it's not an electrical connection.

The switch in the combo valve is normally open. There is a shuttle valve that normally stays in the centered position (pressure from the front brakes is applied to one side and pressure from the rear is applied to the other ... it balances out and doesn't move the valve). But if the front or rear brakes develop a leak, the resulting pressure loss in that half of the system causes the shuttle valve to move off-center and trip the switch. When tripped, the electrical contact is grounded to the body of the combo valve. That completes the circuit for the "BRAKE" warning light on the dash. Many cars also use the same warning light for the parking brake. In that case, another switch on the parking brake assembly completes the circuit to ground.

I think most new OE style combo valves run around $80 to $100 (maybe a bit more or less).

If you change to rear discs later on, you shouldn't have to run a new front-to-rear line or change the Tee at the rear axle. But you will have to change the combo valve to one meant for a disc/disc system. I believe the disc/drum combo valves have a residual pressure valve built in that holds a little pressure on the rear brakes. The front/rear proportioning is likely a little different too. And if the master cylinder you're using now doesn't have a very large reservoir for the rear brakes, it would be a good idea to change that out for one meant for a disc/disc system too.

Thanks. That's what I see with the combo valve, the rubber cap with the pin sticking thru.

4. I'd recommend following the basic layout that GM used in the late 70's and early 80's. Run a line from the MC front outlet to the combo valve front lnlet. Run another line from the MC rear outlet to the combo valve rear inlet. These lines can be fairly long if you want to hide the combo valve down along the frame. The combo valve has 2 front outlets. Normally, you'd run a line from each front outlet to each caliper flex line. But with a line lock, you can plug one of the combo valve front outlets. Run a line from the other front outlet to the line lock. Then put a Tee fitting after the line lock to split off to the left & right calipers. For the rear, run a single line from the combo valve rear outlet back to the flex line. The combo valve already contains a proportioning valve but you can add the adjustable one into this line if needed. On the rear axle, put a Tee fitting at the end of the flex line to split off to the left and right wheel cylinders.


may be a stupid question, but does plugging one side of the combo valve bother the meter valve operation of it for the front brakes. It seems that with 2 outlets, this valve should send equal pressure to both front brakes. I guess with one side deadheaded, it will send all the pressure to the one. correct? It just seems with 2 outlets that they must have did that for a reason. One thing I did notice though was the line from the front of the MC going to the Combo valve and the lines going to the front brakes are the same size. But they are smaller than the line on the back of the MC to the combo valve and then out to the back brakes. Is this because of the longer run, more pressure loss?

I was looking through some pictures/diagrams I had saved and I had one that showed exactly what you described. I had another though that showed the linelock after the Master Cylinder and before the combo valve. Does that work?

Hi siderman,

The two front outlets are Tee'd together inside the combo valve. So plugging one and connecting a Tee fitting off the other won't cause any problems. In fact, the front brake lines on 71 & 72 GM pick-up trucks were hooked up like that from the factory. I'm guessing they did it that way to simplify the line routing.

All of the front brake lines in/out of the combo valve are 3/16". As you've noticed, GM used larger 1/4" lines between the MC & combo valve and from the combo valve back to the rear axle. They dropped back to 3/16" on each of the rear axle Tee to wheel cylinder lines though.

There have been a few rather lengthy discussions here about fluid pressure and volume in 3/16" vs. 1/4" brake lines. I don't think it really makes any difference being a closed system so I'm not sure why GM went with 1/4". Older GM vehicles with single reservoir master cylinders used 3/16" for the front-to-rear line. And I believe Chrysler & Ford both used 3/16" on all their vehicles. In your case, I'd recommend using 1/4" line for those sections simply because it'll match up to the master cylinder & combo valve. Just be sure to use a flex line for the rear axle that'll accept a 1/4" line on the inlet.

I've seen line lock installation diagrams showing it placed between the MC & combo valve too. The only problem I can see with that is it has the potential to trip the shuttle valve (brake warning light switch) in the combo valve. Under normal conditions you have the front and rear pressure balancing out on each side of the shuttle valve keping it centered. But if you hold pressure on the front (via the line lock) and release pressure to the rear (by letting off the brake pedal), it'll move the shuttle valve, turning on the brake warning light. By placing the line lock after the combo valve, it won't be holding pressure on the front of the shuttle valve. It'll just be holding pressure on the calipers and lines after the combo valve.