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Username Post: Self adjusting brakes        (Topic#351527)
Posts: 170

Loc: San Diego
Reg: 06-29-15
06-14-18 08:12 AM - Post#2736942    
    In response to junky


these are all great suggestions, I will try to upload pics hopefully after work today.

To answer some of the questions:

1. Yes, I rebuilt brakes to factory specs, hoses, wheel cylinders, shoes, OEM drums turned, etc. With new replacement parts.

2. The MC is manual, no booster.

3. I sold the oem master cylinder and separated the front and rear brake lines at the junction box for safety, so undoing all of that would be a lot of work. Brakes fine after those mods.

4. the shoes were not contoured because shops here in CA don't accept those jobs - at least where I went. I beveled the ends of the linings with a file.

5. The pedal travel and parking brake travel, and stopping power is all satisfactory IF all star wheels (verified correct placement) are adjusted to a slight drag on the wheel.

The rear brake hardware limits the adjustment at a point where the pedal travels about 6-7" to stop.

IF GM intended it that way, then all is working as it should. I just expected this self adjusting hardware to expand the shoes to leave pedal travel at about 3-4" which I liked and CAN be achieved IF I adjust star wheels myself, which is OK but a little more involved than the self adjusting feature that I was hoping for.

So is brake pedal travel of about 6-7" normal for self adjusting brakes? I just don't have access to any original set ups to compare mine to....

my next step is to mark the star wheels with a scribe and see if the mark rotates or not using the self adjusting feature.

thanks again!

61 Impala bubbletop, 283, 2bbl Rochester, generator, points and drum brakes- daily driver
'49 Harley Davidson FL - Panhead


Bel Air kiwi 
"3rd Year" Silver Supporting Member
Posts: 4293
Bel Air kiwi
Loc: New Zealand
Reg: 04-24-14
06-18-18 11:30 PM - Post#2737467    
    In response to YOUNG57

Hi Guys, Just to correct some miss information here.

"Is the dual master cylinder you installed for drum / drum brakes or for disc / drum brakes. Disc / drum master cylinders have an internal proportion valve that gives the rear brakes a different bias.

Drum / drum dual master cylinders do not use internal or external proportioning valves. Drum / drum brakes rely on the larger front and smaller rear wheel cylinders and shoes to proportion the braking bias."

There is no such thing as a drum or disc master cylinder. A master cylinder just has a bore size. With some extremely rare exceptions the bore is the same diameter all the way through. All the early masters were usually singles and usually underfloor. Mostly they worked on four wheel drum brakes and so had no need whatsoever for any type of proportioning valve.

I have never seen an internal proportioning valve. Some late model pre ABS masters did have them cast into the side but that is very much an OE idea.

What all early Chevy single master cylinders had from day one was an internal RLP valve. Residual Line Pressure Valve.
This was necessary as the wheel cylinders were above the underfloor master. It normally held about 10psi in the drum lines to hold the wheel cylinder cups up and in position so they did not allow air ingress when the brake springs pushed the cylinders back to rest. Syringe effect.

The single master kit shows an early RLP valve in the bottom Rh corner. (Not a chevy one.)
You can only put them internally on the last chamber of an inline exit master.
So it is very easy on a single master but only one RLP can ever be fitted to a dual master internally.

If a dual has to have two RLP valves one can be internal, but one must be external. However if it is side outlet they will both be external as seen in the first image.

If you have a firewall mounted master you don't absolutely have to have RLP valves but it is like going into a gunfight without your pistol cocked.

Most early firewall mount masters from outside the US just used an internal RLP on the furthest chamber for the rear brakes and used gravity to do the front discs. Often the rear fill chamber was larger as well, but the bore sizes were the same. The lack of need for a disc RLP comes about because the caliper pistons are much larger in area than typical wheel cylinders and so only require the very lightest of pressure to make them stay in light application.

If the master is staying underfloor then the disc line should have a 2psi RLP.

There are various ways of balancing front to rear brake bias in a vehicle and this is done very carefully and with repeated testing on a large experience base at the factory. In the Drum/Drum case this may be wider front shoes, more wheel cylinder, different diameter wheel cylinder, different lining areas, etc.

However the line pressure in the system is generated by the pedal ratio through the master and into the lines by the driver effort. So the maximum line pressure is unique to each driver as is the traction in each braking situation.

This would normally mean an operating range of 10 psi to say 750 psi for drums. With master bores normally 1" or smaller on similar sized wheel cylinders. Larger diameter single wheel cylinders on front, or paired ones give more front braking effort than rear to achieve front bias in the balance.

The way the shoes engage can also be significant and early cart style brakes like the Huck had only one shoe being drawn into the drum by braking effort. The Bendix system introduced in the early 50's changed that to two self energising shoes and made between 10-15% more stopping power with the same master pedal and wheel cylinders. Later they were also made self adjusting.

Drum/drum systems can benefit from a small brake booster in the 1.2-1.4 ratio range. The extra couple of hundred psi does help stopping, reduces effort, and avoids overpowering traction.
Some late 50's premium cars had Brake boosting as an option. Still usually with a single master.

When disc brakes were transferred from aircraft to high volume production vehicles in the early 60's there was an immediate issue with the required line pressure to make the new system work versus what was needed before. Now line pressures from 2psi to 1500 psi were needed to take advantage of the new configuration. The big caliper pistons, smaller pad area, and fast heat dissipation of the open rotor meant a doubling of pedal effort would be required to take advantage of the new systems.

As this was both physically difficult to achieve and not what had gone before the pedal pressure efforts intentionally remained similar but the line pressure was artificially boosted by a ratio of about 2:1. This double line pressure was completely incompatible with the operating range for existing rear drums. Rear brake lock up/failure was inevitable even under mild situations. So one of two fixes was applied.

Outside of the USA the standard fix was to reduce the rear wheel cylinder dramatically in diameter to reduce the area and restore function. Small wheel cylinder with high pressure = big wheel cylinder low pressure. So double the pressure on half the area is the same maximum effect. Please note this is area reduction, not half the diameter.

However in the US market the predominance of drum/drum and disc/drum vehicles in the same platform lead to a dilemma of how to make a safe version that would not be mixed up in service to cause safety liabilities.
(This is why I believe you got fixed proportioning valves.)

The same master could be used for both disc/drum and drum/drum cars. The same rear wheel cylinders and all rear braking components were interchangeable. It was simply a matter of making the booster deliver sufficient line pressure for the discs and then prevent the high limits from being applied to the rear drums. This is what a fixed proportioning valve does. It simply changes/limits the maximum pressure to the rears and sometimes the gradient of application.

Each combination valve is vehicle specific and it may also feature failure warning systems and an idea called a moderating valve which slows the early application of the discs to allow the less effective drum rears to start working. This is about driver feel not braking performance as it limits the feeling of the rear trying to come around under braking.
Personally I doubt most parts houses have any idea what they are really getting out of China and what it would actually work properly on.

The second type of proportioning valve is the adjustable type and this is far more useful in bespoke work as it can be adjusted to give quite a range of braking balance and feel, particularly when there is a large mismatch off braking front to rear caused by high line pressures, or mismatched components. Say Huck rears with disc fronts etc. It's tuneable and some race drivers do this on the move.

The third and somewhat rarer proportioning valve is the variable or load sensing type used in some light commercial vehicles with hydraulic brakes. Some pickups and vans have a variable proportioning valve on the rear brake line which measures the rear axle position through springs or levers. If the suspension is high then it further reduces brake line pressure in response to a low load easy skid situation and if the rear axle is low then it allows full pressure to the rear of the heavily loaded vehicle which maximises its stopping, rather than a fixed one size fits all middle setting.

A Vehicle with disc fronts or all discs is going to require a large diameter booster to get a good boost ratio plus a strong vacuum signal also. This may have to be artificially created off the engine where extreme cams and supercharging is present.

The third solution used internationally and would work with US rear ends is to only boost the front lines for the discs. There are some US inline boosters but they are not serviced, the English VH40 and 44 were used on a wide range of vehicles and strongly in the aftermarket to boost all drum vehicles with single masters as well. They are more expensive than inline boosters but can go in the engine bay, under the dash, and even in the inner guard with an improved filter. The advantage being if you are never going disc rear you can simply put a dual master underfloor and retain all the stock pedals etc

My rule of thumb for boosters is if it will fit under-floor it's only enough for drums. Please also note that the OE market measures boosters at a vacuum level their engine creates and compare diaphragm diameters not housing diameters.
An aftermarket 6" is an OE 5.5" and the aftermarket rates its boosters at a really high vacuum level of something like 23"Hg.

Cheers Kiwi

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Edited by Bel Air kiwi on 06-19-18 01:40 AM. Reason for edit: No reason given.


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