Brakes - Wives Tales & Misconceptions

5851a said:
From an 81-shop manual they have a slope chart covering all the fox platform cars for the combination valve. Since all of the cars use the same brakes with exception of the wagons. Seems though they concentrated on the amount of weight of the rear of the car. Some of the cars used different size rear wheel cylinders. 1/16th diameter difference does matter. Some of the cars used a different combination valve.

My 95 truck and 91 Escort wagon had a brake valve hooked to rear suspension. I assumed it was a load sensing valve to hold off brakes empty and apply more pressure when loaded. When you start messing with brakes it takes a lot of experimentation.

Dual master cylinders and hydro boost are not the end all cure all. Brakes are a science; I didn't do great in science. Figured Ford's glass house had lot more smarts and time than me.
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The sizing of the actual service brakes depends on the car weight on either axle. That is where friction material, size and MC/caliper/wheel cylinder bore sizes also come into play. That basic design will bring the vehicle down other than a full panic stop.

That is where the PPV comes in. As the vehicle slows, weight is transferred to the front axle and the rear axle becomes light, allowing the rear brakes to lock. When they lock, especially on a wet road surface, the rear of the vehicle may come around.

Where the real problem comes is when an enthusiast will swap brake designs from different vehicle(s) onto his project. Everything is thrown off. This is where an ADJ PPV (or anti-bias valve) comes into play as the OEM PPV is not calibrated for the mix-max. It is really a guessing game unless thought out and a skid-pad is used.

Even a tire size change or vehicle lowering/raising can/will change the factory PPV settings for the car.

On an OEM PPV, KNEE POINT is when the applied pressure to the rear brakes begins to be restricted and the slope is the degree of further pressure restriction according to the level of the hard/panic stop. On an ADJ PPV, only KNEE-POINT is adjustable, slope is not.

A HYDRO-BOOST install will generate greater brake line pressure(s) than say a vacuum booster.

I am a firm believer in a dual reservoir MC as when I was young I went over an embankment in a 62 IMPALA (conv yet with the top down) after the MC decided to quit. It took months of recovery and physical therapy to rid my self of the induced trauma of sucking up two feet of seat cover. Luckily the cover material was cheap vinyl rather than cloth ... :eek:
 

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Somewhere in my past I saw one of those “height sensing valves”. I think it was a Saab. No surprise with the front engine/drive combo with a lot of nose weight. Not to mention the aircraft engineering base of the company. This was after the 2 stroke engines IIRC.
 
- AND IT JUST KEEPS GOING ON -

The answers are all over the board I will add one more as was told to me by a rebuilder that did power brake units for our auto parts store. A residual check valve is used in disk brakes to keep slight pressure on pads since they always have contact on the rotors and have no springs to pull the pads back. He also said drum brake MC's do not have a residual check valve. This is what I have been told and used this practice in all of my builds over the last 50 plus years. Just something to think about
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SOURCE - https://www.jalopyjournal.com/forum...of-the-residual-valve-in-drum-brakes.1220793/

(POSTERS NAME WITHHELD TO PROTECT THE INNOCENT)​
 
The other common clearance problem with 4th gen T-Bird dual reservoir conversions is the under hood brace. The brace on the MC side is slightly different to clear the relatively short factory MC, but longer dual reservoir MC's typically still don't fit and it's common for T-Bird owners to go on a quest for something will fit...or it's common to cut a notch out of the driver side brace, which I definitely won't do. The other thing people do is buy modified braces. I don't know who makes them or if they're a cottage industry, but I know they have them for '66's, and maybe '64-5 too.
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Here is a possible alternative for the MC -

503600


-ABS BRAKES -

503602
 

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I was having trouble with my brakes on my 56 T-Bird. I got a Disc. Brake kit from Hill's in southern Ohio. Very nice kit all the brake lines come with it. The only thing I had to buy was brake fluid and a brake safety switch. When I remove the old master cylinder it ws suppose to have 4 bolts holding it on. Two bolts just stuck in nothing holding them and the other two finger loose. When I push the brake the master cylinder would push out about a inch. The brake fluid was black in color, not good. No wonder I had a poor brake. I installed the Hill's brake kit and wash out the brake lines with brake clean and air pressure. Then what type brake fluid ti install. Dot 3,Dot 4 or Dot 5. Well I did install Dot 5. It's purple in color. Several talk against it for different reason and a couple that use it love it. We will see how it will work. My brake peddle seams firm but it snowed here and 23 degrees not good for a road test. Maybe next week and will let you know the results how the brakes work.Thanks for all your advice and help.
 
When dual reservoir master cylinders originally made it onto the scene, one of the reservoirs was much larger than the other. This was generally considered the reservoir for the drum brakes due to the sheer volume of fluid needed to push the brake shoes out followed by the return of all that fluid back to the master cylinder. Disc brakes didn’t require the same volume in the reservoir. Today, most master cylinders have the same size reservoir as well as masters sharing the same reservoir. These are still considered dual reservoir systems because the internals are separated front to rear.
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SOURCE - https://techtalk.mpbrakes.com/master-cylinder-2/single-vs-dual-reservoir-master-cylinders

Does anyone else see the mistake made in the above description or is it me just being overly sensitive?
 
KULTULZ said:
The sizing of the actual service brakes depends on the car weight on either axle. That is where friction material, size and MC/caliper/wheel cylinder bore sizes also come into play. That basic design will bring the vehicle down other than a full panic stop.

That is where the PPV comes in. As the vehicle slows, weight is transferred to the front axle and the rear axle becomes light, allowing the rear brakes to lock. When they lock, especially on a wet road surface, the rear of the vehicle may come around.

Where the real problem comes is when an enthusiast will swap brake designs from different vehicle(s) onto his project. Everything is thrown off. This is where an ADJ PPV (or anti-bias valve) comes into play as the OEM PPV is not calibrated for the mix-max. It is really a guessing game unless thought out and a skid-pad is used.

Even a tire size change or vehicle lowering/raising can/will change the factory PPV settings for the car.

On an OEM PPV, KNEE POINT is when the applied pressure to the rear brakes begins to be restricted and the slope is the degree of further pressure restriction according to the level of the hard/panic stop. On an ADJ PPV, only KNEE-POINT is adjustable, slope is not.

A HYDRO-BOOST install will generate greater brake line pressure(s) than say a vacuum booster.

I am a firm believer in a dual reservoir MC as when I was young I went over an embankment in a 62 IMPALA (conv yet with the top down) after the MC decided to quit. It took months of recovery and physical therapy to rid my self of the induced trauma of sucking up two feet of seat cover. Luckily the cover material was cheap vinyl rather than cloth ... :eek:
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Height sensing valve....I hear that some of the late 1970's early 1980s Rolls-Royce's had a brake setup that was tied to the suspension. I wonder if this is something like that.
 
The reference I made in post 62 above was like that. It was many years ago, but the valve was on the frame, and the rod went to the rear axle. A simple beam as I remember, typical for an early FWD vehicle. How well it worked I don’t know, but obviously there would be a difference in rear pressure as the chassis shifted under braking. How well that compensated for dry vs wet I don’t know. Modern antilocks make all that moot.
 
Gene F said:
Height sensing valve....I hear that some of the late 1970's early 1980s Rolls-Royce's had a brake setup that was tied to the suspension. I wonder if this is something like that.
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Here is a brief explanation of a HEIGHT/WEIGHT SENSING PPV -

Height-Sensing Proportioning Valves

Some vehicle’s proportioning valves go one step farther, as the knee-point on the graph can vary with the amount of weight on the rear axle. Effectively, as the rear axle weight increases, a linkage between the axle and the body is compressed. This linkage acts on a cam inside the proportioning valve which increases the preload on the proportioning valve spring. The end result is that more rear braking (bias) is allowed as weight is added to the rear axle, helping to take advantage of the increased traction now available at the rear tires. Figure D illustrates this relationship quite clearly.
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FULL EXPLANATION HERE - https://www.brakes-shop.com/brakepedia/general/brake-proportioning-valves

A TRIM-HEIGHT SENSING VALVE is also used on air ride cars so they can be confusted easily.
 
Allow Me To Add This Information Here For Future Reference -

- BRAKE VALVING MISC FACTOIDS - PDV​

One myth that often pops up during conversations is the misconception that if the spool in the pressure differential valve gets (PDV) pushed off-center due to (for example) a brake line failure, it seals off the bad side, allowing you to still have brake pressure on the good side. This is incorrect! It is merely a warning system.

The ends of the spool are turned down to allow brake fluid to pass when it is pushed to one side. If the spool was designed to cut off the brake fluid to the side where the pressure has dropped it would seal the fluid between the master cylinder and the distribution block. This would cause the brake pedal to be rock hard and you would have no brakes at all because the trapped fluid would not give. The dual master cylinder is the safety feature that allows pressure at the good end.

There is nothing inside the pressure differential valve to replace when rebuilding except for two special o-rings and a crush washer.
 
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