Technical Some brake questions !!!

Fixing a few problems with my 27 T touring.
Moving on to the brakes now .Car has 78 chevette calipers a volvo rotors , 42-48 rear brakes and a 40 s master cylinder , firewall mounted .
Shoulds I run residual valves ?
Proportioning valve ?
My father put this stuff on the car in the late 70s early 80s and I am going to leave it this way .It is a v8 60hp

 

It should have a 10 LB residual valve for the rear drum brakes and a 2 lb for the front discs. An adjustable proportioning valve is run by most hot rods.

 

If you know what master cylinder it has find a diagram of the master cylinder. Most all of those single piston master cylinders had a built in residual valve that may or may not hold more pressure back than the disk calipers need. That might require removing the original residual valve out of the master cylinder and putting a spacer washer behind the spring in it's place and putting a 2 lb in the front line and 10 lb in the rear line.
Usually you need the adjustable proportioning valve when you have brakes on the rear that have equal or higher stopping power as the front = disk/ disk or disk or drum front with big drum brakes on the back. That balance the front/ back braking so it doesn't cause issues otherwise.

 

I thought firewall mounted masters did nit require residual valves?

 

Unless the master cylinder has residual pressure capability, a residual valve is ALWAYS needed for a drum brake circuit, irrespective of where the master cylinder is located.

If the master cylinder us below the calipers, a residual valve is needed to prevent excessive drain-back, and excessive piston retraction (which increases reaction time). If the master cylinder is above the calipers (like on the firewall) a residual valve is not needed in a disc brake circuit.

 

My concern here is that the 1940's master, being a drum brake, single circuit master cylinder, it probably does not deliver enough volume to properly actuate the front calipers, and thus you are likely effectively running on rear brakes, only.

 

Would a 1946 ford master have a risidual valve ? Original car would be 4 wheel drum .

 

Calipers are quite small , single piston , maybe 1 3/4 - 2 inch piston .

 

If I remember correctly, the bore of a 1948 master cylinder (that goes with the rear brakes) is 1-1/16".

The master cylinder bore for a 1978 Chevette is 3/4".

Those two numbers are not even close.

 

They are 1-7/8".

I appreciate your desire to leave this as-built, but it is not likely to work particularly well.

 

how well does it stop?

 

Break fluid will go where it is the easiest to go all cylinders will see the same amount of pressure. If the master cylinder is not big enough the pedal will simply go to the floor. With Lockheed brakes not being self energizing you may not need a proportioning valve. I would try it if the back brakes lock up before the fronts and the valve. Caliper pistons are larger but they normally don't travel as far as wheel cylinders to apply.

 

At what point did I mention pressure?

 

Calling close air support: @Kerrynzl @Ned Ludd

 

Several of the above posts make some very good points. Certainly, the combination on this T is unusual, but not unmanageable. The suggestion that the M/C residual valve be removed and replaced by appropriate residual valves plumbed into the front and rear seems like a logical beginning.

Next, doing some stopping tests to determine the relative effectiveness of the front and rear systems in an emergency stop would be useful. If the back brakes do not lock up, either first, or not at all, that would be worth knowing. If they do lock up first, then some method of changing the balance between front and rear is indicated. That could take several forms, i.e. wheel cylinder size in rear or caliper change in the front or master cylinder or some combination of these factors.

I think the current setup in the front, appearance wise, is subtle and worth trying to retain.

I also think the idea that most hot rods have proportioning valves is due to the wide spread notion that they are always, or usually, needed. I think that is uncertain, at best. I would submit that even in those circumstances in which some pressure differential is needed, that could be accomplished by adjusting the sizes of brake components to achieve the desired balance. If the first resort is to a proportioning valve that is akin to 'treating the symptom, not the cause'.

 

I agree with Gimpy on this one, in all of his above posts. There are some seriously mismatched parts here.

I have just about given up on trying to help with brake problems, some people refuse to believe some things.


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Although I havent driven the car in years and years , I dont remember the car being hard to stop or some sort of concern with braking .

 

Isnt mismatched parts what hot rodding is all about ???

 

It is, and when it is done wrong, it can be fatal.

 

If the brakes do not work 100 % it will not see the road till they do !!!!

 

yes....providing the 'mismatch' yields an improvement in performance, not a decrease in performance and/or safety.

 

I will say that I don’t agree residual valves are needed if the master is on the firewall, depends on wheel cylinder construction, they are not needed with wheel cylinders that have cup expanders.


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Lets put it this way , my original T has a transmission brake and thats it !!!!
No front brakes no rear brakes .
Suprisingly the transmission brake will lock up the rear tires LOL

 

Then it sounds to me like you are already convinced that they are fine, so why ask if you should change anything?


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Looking for suggestions for improved or safer operation !!!!

 

Your current master cylinder is right in-between what GM typically uses for around 3" (2.94) pistons, for manual brakes (1") and power assist (1-1/8").

Your caliper pistons are 1-7/8".

We already know that the bore of the master cylinder (sets the pressure) is okay for the rear brakes, as it is matched to the type of brakes. That leaves the calipers as the variable (master cylinder volume aside)

Short of bumping the caliper bore to better match the master cylinder bore, I am not sure what to do here.

Changing he master cylinder bore will make it incorrect for the rear brakes. If you dropped the master cylinder bore to 3/4", to match the front calipers, you'd be facing backwards at the first hard stop.

Do you know what year the Volvo rotors are, or how thick they are?

I would swap the master cylinder for a dual circuit one, that is set up for disc drum, with the same bore. Then, I would change the calipers to GM "metric" ones (with new brackets), possibly with pad spacers, to fit the Volvo discs.

That way, all of the bores/pistons/cylinders are fairly matched. From there, you can assess of you need a proportioning valve.

 

Thanks for sticking with me on this Gimpy.
My only intensions on using this set up is that my Dad built this car as an original T in the 60s
He changed it in the 70s to a v8 60 hp with 39 rear end and a 39 front end and later installed the brakes .As I said I dont remember what it braked like , but drove the car plenty , as he did as well.
If I cant or dont feel the brakes are up to snuff , I will put the 42 front drums on the car .

 

That would probably be the easiest, and period-correct, also likely a bolt-on, depending on exactly what spindles are on there.

 

We're not needed here.........You are doing fine on your own

To the OP ...
A stock standard Chevette would "out brake" your T [so if you set that as a safety benchmark it would be a step in the right direction.

It scares me the thought of Single M/C's but I have a 57 Chevy that is the same.
Just make sure ALL your hoses and hard-lines are better than good condition if you stay with the single M/C
With a firewall mounted M/C you don't need a residual valve for the callipers [only the drums need them]

Now to get the rear proportioned to the existing front callipers, You would need 9" Drums and 3/4" wheel cylinders [ your existing rears are 12" drums and 1-1/8" wheel cylinders]
So you would need to change the M/C to 7/8" and the rear drums and backing plates, which ends up totally re-engineering your car.

You have basically given yourself the correct solution! Change the front brakes to compatible Ford brakes [12" drums and 1-3/8" Wheel cylinders]
It will out perform the skinny tyres you have ,and still look period correct.

Cool car by the way!

 

I read and understand what you guy's are saying . I Thank you for your information .
I do have to try this and see if I can make it work or at least jog my memory as to how it did work !!