Brake Master Cylinder Bore Sizing

[Gary Lewis]

Jonathan/FordF834 and I've been emailing back and forth about master cylinders, hydroboost, proportioning valves, etc. He said he'll start a thread after while, but I've done some reading, measuring, etc and want to get started.

One of the issues we discussed is that Jan's rear brakes lock up early and Jonathan's front brakes lock up early. So the question was what difference does a larger or smaller master cylinder make.

So I did some reading and found that this article on How Master Cylinders & Combination Valves Work is informative. But the key sentence to me is "If the brakes are operating properly, the pressure will be the same in both circuits." And that seems to say that a different master cylinder won't make one end or the other slide before the other.

Further reading says that too small of pistons in a master cylinder will cause a soft-feeling pedal with a lot of travel because you have such a small volume of fluid displaced with each 1" of pedal movement. On the other hand, too large of master cylinder pistons cause a very hard pedal with a very quick application of the brakes with a small movement of the pedal. But, in neither case will it cause one end or the other to apply first.

And that lead to a question was about the valve in the system down below the driver's feet, and what it does. Some call it a "proportioning valve", but Ford calls it a "pressure differential switch". And that's all it does - turn the light on if the pressure in one circuit or the other, meaning front or rear, gets far higher than the other. In that case a shuttle gets pushed one way or another and causes a connection to be made to ground, which turns on a warning light. But it doesn't change the pressure in either circuit - it just reports that there is a differential in pressure.

Another thing we talked about is the bolt spacing where the master bolts on. I just happen to have the old master off of Dad's F150, the one off of the '90 F250 called Huck, and the 90's F450 that Jim parted out and that had hydroboost. Turns out that the lighter trucks have a bolt spacing of 3.2" where the master bolts to the booster, and the big truck with hydroboost has a spacing of 3.44".

Ok, those are my understandings. What am I missing? What did I state incorrectly? Where am I wrong?

 

[Ford F834]

Gary, not only did you beat me to it but you did a far better job of outlining and researching the issue!

So if I understand it right, the master cylinder’s job is to apply equal pressure to both lines, the pressure differential valve is just a warning sensor.

I’m not sure then, how (or even if) the janky load sensing valve on the 8-lug trucks adjusts the rear braking. Likewise for the adjustable aftermarket proportioning valves. According to the article, the amount of front vs rear braking is inherent in the in the design of the wheel cylinder and pad/rotor/drum surface.

The good news, I suppose, is that the brake system should respond the same with any master cylinder (except for pedal feel). The not-so-good news is there does not seem to be any way to control/correct difference in front to rear braking.

I did some test stops in my driveway. I am not sure about my panic stop pavement skid marks when I nearly hit a cow, but on dirt I can clearly see that my rear brakes are locking up first just like Jan’s truck:

Just for the record, I put brand new calipers, wheel cylinders, pads and shoes when I converted to hydroboost. I purged out all the bubbles as well as discolored brake fluid. The truck stops well, and pulls nice and straight. But I am experiencing premature ~rear wheel lock-up, and I get brake fade/burning smell rather easily on steep down grades even without a load. Time for a new master cylinder?

[Rembrant]

And that lead to a question was about the valve in the system down below the driver's feet, and what it does. Some call it a "proportioning valve", but Ford calls it a "pressure differential switch". And that's all it does - turn the light on if the pressure in one circuit or the other, meaning front or rear, gets far higher than the other. In that case a shuttle gets pushed one way or another and causes a connection to be made to ground, which turns on a warning light. But it doesn't change the pressure in either circuit - it just reports that there is a differential in pressure.

I've been wondering about that thing. On my 84 is on a bracket on the frame almost directly below the master cylinder...next to where the oil dipstick is on my 302. I'm planning on re-doing my entire braking system this winter, including ALL lines. Wasn't sure what to do with this little device...replace or re-use. I don't remember seeing any wires going to it, but I haven't looked at it closely either. My truck isn't here right now so I can't even check it. Hopefully I can just clean it up and re-use it.

 

[85lebaront2]

And that lead to a question was about the valve in the system down below the driver's feet, and what it does. Some call it a "proportioning valve", but Ford calls it a "pressure differential switch". And that's all it does - turn the light on if the pressure in one circuit or the other, meaning front or rear, gets far higher than the other. In that case a shuttle gets pushed one way or another and causes a connection to be made to ground, which turns on a warning light. But it doesn't change the pressure in either circuit - it just reports that there is a differential in pressure.

I've been wondering about that thing. On my 84 is on a bracket on the frame almost directly below the master cylinder...next to where the oil dipstick is on my 302. I'm planning on re-doing my entire braking system this winter, including ALL lines. Wasn't sure what to do with this little device...replace or re-use. I don't remember seeing any wires going to it, but I haven't looked at it closely either. My truck isn't here right now so I can't even check it. Hopefully I can just clean it up and re-use it.

The block on Darth had a brake failure warning switch, if either system (front or rear) lost pressure it would illuminate the brake warning light. It also lit up with the parking brake applied.

[Gary Lewis]

And that lead to a question was about the valve in the system down below the driver's feet, and what it does. Some call it a "proportioning valve", but Ford calls it a "pressure differential switch". And that's all it does - turn the light on if the pressure in one circuit or the other, meaning front or rear, gets far higher than the other. In that case a shuttle gets pushed one way or another and causes a connection to be made to ground, which turns on a warning light. But it doesn't change the pressure in either circuit - it just reports that there is a differential in pressure.

I've been wondering about that thing. On my 84 is on a bracket on the frame almost directly below the master cylinder...next to where the oil dipstick is on my 302. I'm planning on re-doing my entire braking system this winter, including ALL lines. Wasn't sure what to do with this little device...replace or re-use. I don't remember seeing any wires going to it, but I haven't looked at it closely either. My truck isn't here right now so I can't even check it. Hopefully I can just clean it up and re-use it.

Last I saw the pressure differential switch was going for about $100. And since it just reports a difference in pressure, via a 1-wire hookup, I'd use it if it works.

[Gary Lewis]

Last I saw the pressure differential switch was going for about $100. And since it just reports a difference in pressure, via a 1-wire hookup, I'd use it if it works.

Ok folks, time for full disclosure: I was WRONG! Twice!

First, the circuit to the Dual Brake Warning Switch, which resides on the Pressure Differential Switch, is a 2-wire arrangement as it passes through that switch and is grounded in the ignition switch.

Second, the 1980 factory shop manual says:

The pressure, differential, proportioning and metering valve are three different systems. (Figs. 4 and 5). There are three functions of the pressure differential valve: It delays full effectiveness of the front brake, it proportions pressure to the rear system, and it operates a warning system in case of malfunction. During heavy brake application, the proportioning valve reduces pressure to the rear brakes to avoid lock-up. The metering valve delays effectiveness of the front disc brake until the rear brake shoes are against the drum.

So, a bad valve, or the wrong valve, could easily cause the rear lock-up problem that both Jonathan and Jan are having.

I'd thought that maybe by going hydroboost the valve was now the "wrong" valve. But if the front and rear brakes are still the original size the original valve should work perfectly regardless of how the boost is derived.

Anyway, I've also updated the webpage on Brakes.

[85lebaront2]

Ok folks, time for full disclosure: I was WRONG! Twice!

First, the circuit to the Dual Brake Warning Switch, which resides on the Pressure Differential Switch, is a 2-wire arrangement as it passes through that switch and is grounded in the ignition switch.

Second, the 1980 factory shop manual says:

The pressure, differential, proportioning and metering valve are three different systems. (Figs. 4 and 5). There are three functions of the pressure differential valve: It delays full effectiveness of the front brake, it proportions pressure to the rear system, and it operates a warning system in case of malfunction. During heavy brake application, the proportioning valve reduces pressure to the rear brakes to avoid lock-up. The metering valve delays effectiveness of the front disc brake until the rear brake shoes are against the drum.

So, a bad valve, or the wrong valve, could easily cause the rear lock-up problem that both Jonathan and Jan are having.

I'd thought that maybe by going hydroboost the valve was now the "wrong" valve. But if the front and rear brakes are still the original size the original valve should work perfectly regardless of how the boost is derived.

Anyway, I've also updated the webpage on Brakes.

Gary, the other item that can contribute to rear brake lockup is the brake load proportioning valve. It varies the pressure for the rear brakes according to the ride height of the rear suspension. If it is not working at all it may allow full pressure to the rear brakes, and may also, if not properly adjusted if disconnected and reconnected have the pressures wrong. If the truck has been lifted, it may reduce the rear pressure to the point the front brakes lock up first.

I discovered in doing the upgrades on Darth, that in addition to different size master cylinders, the rear brakes have different size cylinders. In that case it becomes smaller = faster application and shorter pedal stroke, but less power applied to the shoes, larger = slower application and longer pedal stroke, but more power applied to the shoes.

I have had my rear brakes lock, but not often and only empty if I really jump on them hard. Never had the fronts lock except on ice and snow.

[Ford F834]

Ok folks, time for full disclosure: I was WRONG! Twice!

First, the circuit to the Dual Brake Warning Switch, which resides on the Pressure Differential Switch, is a 2-wire arrangement as it passes through that switch and is grounded in the ignition switch.

Second, the 1980 factory shop manual says:

The pressure, differential, proportioning and metering valve are three different systems. (Figs. 4 and 5). There are three functions of the pressure differential valve: It delays full effectiveness of the front brake, it proportions pressure to the rear system, and it operates a warning system in case of malfunction. During heavy brake application, the proportioning valve reduces pressure to the rear brakes to avoid lock-up. The metering valve delays effectiveness of the front disc brake until the rear brake shoes are against the drum.

So, a bad valve, or the wrong valve, could easily cause the rear lock-up problem that both Jonathan and Jan are having.

I'd thought that maybe by going hydroboost the valve was now the "wrong" valve. But if the front and rear brakes are still the original size the original valve should work perfectly regardless of how the boost is derived.

Anyway, I've also updated the webpage on Brakes.

Gary, once again thank-you for digging up the answers straight out of the Ford literature 😎

So it sounds like I may have a differential valve problem. And Jan may have a differential valve and/or load sensing valve problem.

One interesting thing I noticed is that many of the F100/150’s use the same differential valve as the F250/350’s. And the 8 lug trucks use the load sensing valves while the lighter trucks do not. I wonder if this is related at all with the 8 lug trucks having dual piston front calipers? The other note is that the load sensing valve was discontinued (and presumably replaced?) with the rear ABS system?

On a related topic, lots of folks out there are doing rear wheel disc brake conversions. Anyone ever hear of proportioning problems when that is done? If I go to replace the pressure differential valve, I wonder if there would be any merit to one of the adjustable aftermarket ones?

 

[Gary Lewis]

Gary, once again thank-you for digging up the answers straight out of the Ford literature 😎

So it sounds like I may have a differential valve problem. And Jan may have a differential valve and/or load sensing valve problem.

One interesting thing I noticed is that many of the F100/150’s use the same differential valve as the F250/350’s. And the 8 lug trucks use the load sensing valves while the lighter trucks do not. I wonder if this is related at all with the 8 lug trucks having dual piston front calipers? The other note is that the load sensing valve was discontinued (and presumably replaced?) with the rear ABS system?

On a related topic, lots of folks out there are doing rear wheel disc brake conversions. Anyone ever hear of proportioning problems when that is done? If I go to replace the pressure differential valve, I wonder if there would be any merit to one of the adjustable aftermarket ones?

Welcome.

I've upgraded the Driveline/Brakes page yet again. This time I've added verbiage and part numbers for the Brake Load Proportioning Valve (BLPV) as well illustrations for both it and the Brake Pressure Differential Valve. See what you think, please.

As for the reason why the heavier trucks got the BLPV, I don't know. Maybe it was the dual-pistons in the front. Or maybe it was the much heavier springs in the rear. Or a combination.

And, that valve was replaced when they went to RABS, apparently in '90. I say that because I have a RABS electronic module from Huck, the '90 F250. The catalog shows that the BLPV was used through '89, so I'd say RABS appeared in '90.

Last, I think an aftermarket adjustable proportioning valve may be just the thing. I believe some of the later master cylinders incorporated the pressure differential valve in them, so if you had one of those and an adjustable pro valve then you should be set.

[Ford F834]

Welcome.

I've upgraded the Driveline/Brakes page yet again. This time I've added verbiage and part numbers for the Brake Load Proportioning Valve (BLPV) as well illustrations for both it and the Brake Pressure Differential Valve. See what you think, please.

As for the reason why the heavier trucks got the BLPV, I don't know. Maybe it was the dual-pistons in the front. Or maybe it was the much heavier springs in the rear. Or a combination.

And, that valve was replaced when they went to RABS, apparently in '90. I say that because I have a RABS electronic module from Huck, the '90 F250. The catalog shows that the BLPV was used through '89, so I'd say RABS appeared in '90.

Last, I think an aftermarket adjustable proportioning valve may be just the thing. I believe some of the later master cylinders incorporated the pressure differential valve in them, so if you had one of those and an adjustable pro valve then you should be set.

Thank-you for the updated pages Gary! That is great info!