Bullnose Enthusiasts Forum

Brake Master Cylinder Bore Sizing

Classic

List

Threaded

30 messages Options

Gary Lewis

Brake Master Cylinder Bore Sizing

Administrator

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?

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

Ford F834

Re: Brake Master Cylinder Bore Sizing

Administrator

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?

SHORT BED 4-DOOR DIESEL: 1986 F350 4x4 under construction-- 7.3 IDIT ZF5+GVOD

STRAIGHT SIX 4X4: 1981 F150 2wd to 4x4-- 300 I6 close ratio diesel T19, hydroboost brakes, Saginaw steering

BIG F: 1995 F-Superduty under construction— converting to 6.9L IDI diesel ZF5+DNE2

Rembrant

Re: Brake Master Cylinder Bore Sizing

In reply to this post by Gary Lewis

Gary Lewis wrote

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.

1994 F150 4x2 Flareside. 5.0 w/MAF, 4R70W, stock.
1984 F150 4X2 Flareside. Mild 302 w/ 5spd. Sold.
1980 F150 4X4 Flareside. 300i6 w/ 5spd. Sold in 2021.
1980 F100 4X2 Flareside. 351w/2bbl w/NP435. Sold in 1995

85lebaront2

Re: Brake Master Cylinder Bore Sizing

Administrator

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.

Bill AKA "LOBO" Profile

"Getting old is inevitable, growing up is optional" Darth Vader 1986 F350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator Wife's 2011 Flex Limited Daily Driver 2009 Flex Limited with factory tow package Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413

Gary Lewis

Re: Brake Master Cylinder Bore Sizing

Administrator

In reply to this post by Rembrant

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, AKA "Gary fellow": Profile

Gary Lewis

Re: Brake Master Cylinder Bore Sizing

Administrator

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, AKA "Gary fellow": Profile

85lebaront2

Re: Brake Master Cylinder Bore Sizing

Administrator

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.

Bill AKA "LOBO" Profile

Ford F834

Re: Brake Master Cylinder Bore Sizing

Administrator

In reply to this post by 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?

Gary Lewis

Re: Brake Master Cylinder Bore Sizing

Administrator

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.

Gary, AKA "Gary fellow": Profile

Ford F834

Re: Brake Master Cylinder Bore Sizing

Administrator

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

Gary Lewis

Re: Brake Master Cylinder Bore Sizing

Administrator

My son and his family arrive in a few hours, so I'll be scarce on here this week as we will have the whole family together under one roof!

Otherwise I'd see if the factory shop manual has some magic words explaining how the load sensing valve works. But if any of you find it please post and I'll try to add them to the page.

Gary, AKA "Gary fellow": Profile

85lebaront2

Re: Brake Master Cylinder Bore Sizing

Administrator

Gary, it wasn't used until 1985, if you have the manuals for Big Blue it might be in there. I do know that there is no given adjustment for them, they come preset for full extension of the rear suspension. When I changed Darth's rear axle, I took the bracket off and lifted it up out of the way while I switched axles.

Bill AKA "LOBO" Profile

Gary Lewis

Re: Brake Master Cylinder Bore Sizing

Administrator

The master parts catalog actually shows a part number for 1984, but it is one-size-fits-all as opposed to the multiple parts used in '85 and later to make them specific to the springs/GVW.

Anyway, I do have the 1985 factory shop manual and will see what it says re them. Since they'd have been new that year surely they wrote something about them.

Gary, AKA "Gary fellow": Profile

Romel

Re: Brake Master Cylinder Bore Sizing

What I have observed on my truck (Jan´s truck) is when the park brake was aplied over night, I release it, the truck is free moving. The first braking, the rear lock always. I have to really be careful to apply brake force to not lock it. Then it gets better, but any panic brakining the rear locks always. I have looked at the proportioning valve, the piston inside sitting in the middle, no light warning. I do have a rear load sensing valve above the rear axle conected, if I disconect it it moves free. The brakes bleed fine, rear, front. I have poured in gallons of brake fluid, flushed the system, no air anywhere.

1986 XLT Lariat F250 6.9 IDI, supercab, sidewinder, psd intercooler, hydroboost

Gary Lewis

Re: Brake Master Cylinder Bore Sizing

Administrator

That's not a good situation, and is very unlike the situation I have on Big Blue. I wonder if someone has "worked on" the ride height sensing valve at the rear. I've found a couple of TSB's on it and will put them on the website and let you know when they are on.

Gary, AKA "Gary fellow": Profile

85lebaront2

Re: Brake Master Cylinder Bore Sizing

Administrator

This post was updated on .

I did find in doing the work on Darth that there are different size rear brake cylinders. If somewhere in the past the rears were changed to a larger size, it can make them lock up easily.

One other item, when you did the Hydroboost change, if you didn't keep the original master cylinder, is the new one the same bore size?

I did run into a similar situation on a 1978 Oldsmobile Delta 88 Royale with the 5.7L Diesel. Gm in their infinite wisdom decided despite the added weight of the heavier block and extra battery to use the V6 running gear under these cars. First problem was a complete failure of the front pads at around 20,000 miles or less (it's been 40 years) the solution, under warranty (TSB) was to upgrade the front pads to the B&C body heavy duty (police package) pads. The other issue though that the change aggravated was the tendency to lock the rear wheels any time you made a hard stop. I solved it by putting the HD rear shoes on the car which were a harder composition.

If your rear shoes are fairly soft, that may be what you are seeing.

Bill AKA "LOBO" Profile

Romel

Re: Brake Master Cylinder Bore Sizing

I did keep the original cast master, I hate it for turning the fluid into rusty poridge but oh well.

1986 XLT Lariat F250 6.9 IDI, supercab, sidewinder, psd intercooler, hydroboost

Gary Lewis

Re: Brake Master Cylinder Bore Sizing

Administrator

This post was updated on .

I'm going with a 1990's F450 master cylinder, which is one of the plastic units with an aluminum cylinder, to replace the cast unit on Big Blue when I do the hydroboost upgrade.

Anyway, I've added two Technical Service Bulletins to the website today:

Brake Ride Height Proportioning Valve

Brake Noise

The first one discusses the rear proportioning valve and issues that can occur with changes to the suspension, but it may not help you, Jan.

Gary, AKA "Gary fellow": Profile

Romel

Re: Brake Master Cylinder Bore Sizing

I just verified it must be the proportioning valve. If u aply slightly the brakes, and then go firm, they work as they should, the front suspension sinking and good stoping power. If u panic brake, the rear locks and the stoping power is limited, u slide like on ice.

1986 XLT Lariat F250 6.9 IDI, supercab, sidewinder, psd intercooler, hydroboost

Gary Lewis

Re: Brake Master Cylinder Bore Sizing

Administrator

When you say "the proportioning valve", are you meaning the one on the frame under the master cylinder? That's the one I think it probably is. But, there's the ride-height sensing one in the rear. Might it be malfunctioning or improperly adjusted?

That one is supposed to limit rear braking with no load. How about disconnecting the arm from the truck, not the valve as the TSB says do not do that, and tie the arm in another position and try the braking? Perhaps you can figure out from that if it is the rear valve or not.

Gary, AKA "Gary fellow": Profile