Anyone try 13/16 wheel cylinders on the rear wheels after converting to front disc brakes? I see quite a few people have done it on the A and B bodies with great success, but haven't really heard much about that being done to C bodies. Would it be better going to 7/8 cylinders?
13/16 wheel cylinders on rear axle for disc brake swap
- Thread starter edbods
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I think the question to ask is why you think you need to decrease the amount of braking to the rear?
Does the new disc brake MC have a smaller piston that would increase the pressure applied?
Does the new disc brake MC have a smaller piston that would increase the pressure applied?
The new master has a 1.031 (1-1/32) inch bore, up from the old 1 inch one. At least on the A and B bodies, the 15/16 cylinders were apparently already notorious for locking up the rears far too easily, and the fix on the police models at least was to just use 13/16 cylinders. Mind you, these aren't the land yachts that C bodies are so...yeah. Wasn't sure if that'd be applicable here.
A larger MC piston will reduce the pressure to the brake cylinders.
The rear of the c-bodies are heavier and the longer wheelbase would also be a factor, so I would say that you don't want to reduce the pressure any more than you have with the larger MC piston.
You should have installed an adjustable proportioning valve as part of the conversion. That means if you are having a rear lock up issue you should be able to reduce excessive pressure to the rears with the adjustable valve.
Dave
Dave
That makes a lot more sense to me.
According to my 1967 Dodge Monaco / Polara service manual:
Wheel cylinder bore (front): 1 1/8 inch
Wheel cylinder bore (rear): 15/16 inch
Wheel cylinder bore (rear, with front disk): 7/8 inch
Master cylinder bore (standard 4-wheel drum): 1 inch
Master cylinder bore (with front disk brakes): 1 1/8 inch
Some of the following comes from an Inline Tube video from 4 years ago, and a RamMan video from 9 years ago:
From 1967 to 1970 for front disk brake cars, a "pressure reducing valve", aka "hold-off" valve was added to the rear brake line about a foot from the main distribution block. If line pressure did not exceed 250 psi this valve did nothing. When line pressure exceeded 250 psi this valve slowed the rate of increase of pressure to the rear drum brakes.
In late 1969 a front hold-off valve aka metering valve was introduced to delay brake action of the front disk brakes to allow the rear drum brakes to energize first. Also in late '69 the rear hold-off valve was combined into the main distribution block.
In 1972 all 3 components were combined into a single distribution / metering block, made of cast iron (not brass).
What's an open question (for me) is if a 4-wheel drum system would benefit from having the rear hold-off valve, and would such a valve prevent rear-wheel lockup during "panic" braking like it's designed to do in a car with front-disk setup.
It's odd - I think regardless if the front wheels were disk or drum, there might have been this idea that it's good to engage the rear brakes first, then the fronts, but just don't go overboard with rear wheel braking force.
Wheel cylinder bore (front): 1 1/8 inch
Wheel cylinder bore (rear): 15/16 inch
Wheel cylinder bore (rear, with front disk): 7/8 inch
Master cylinder bore (standard 4-wheel drum): 1 inch
Master cylinder bore (with front disk brakes): 1 1/8 inch
Some of the following comes from an Inline Tube video from 4 years ago, and a RamMan video from 9 years ago:
From 1967 to 1970 for front disk brake cars, a "pressure reducing valve", aka "hold-off" valve was added to the rear brake line about a foot from the main distribution block. If line pressure did not exceed 250 psi this valve did nothing. When line pressure exceeded 250 psi this valve slowed the rate of increase of pressure to the rear drum brakes.
In late 1969 a front hold-off valve aka metering valve was introduced to delay brake action of the front disk brakes to allow the rear drum brakes to energize first. Also in late '69 the rear hold-off valve was combined into the main distribution block.
In 1972 all 3 components were combined into a single distribution / metering block, made of cast iron (not brass).
What's an open question (for me) is if a 4-wheel drum system would benefit from having the rear hold-off valve, and would such a valve prevent rear-wheel lockup during "panic" braking like it's designed to do in a car with front-disk setup.
It's odd - I think regardless if the front wheels were disk or drum, there might have been this idea that it's good to engage the rear brakes first, then the fronts, but just don't go overboard with rear wheel braking force.
@Davea Lux yeah I knew about proportioning valves and was initially planning on using the 72-onwards mopar distribution blocks that include a prop valve since the lines are in almost the exact same locations, but seeing that wheel cylinders could simply be downsized to get similar results without having to buy an entirely new distribution block/prop valve, especially since I'd need to find something to bolt it to the frame rail, I figured it made sense to just downsize the rear wheel cylinders, especially since they needed replacing anyway due to their age.
No lock-up problems that I can test because the car's still on stands, but once everything's back together (still waiting on firm feel and my steering box lol) I'll do test runs with the current 15/16 cylinders and see what adjustments are needed.
I guess best case scenario is that the stock 15/16s are good (doubtful given mopar man's post about the disc/drum models) all I have to do is get 7/8 or 13/16 cylinders, and worst case, fork out the dough for the >72 distribution blocks then figure out a bracket to bolt it to the frame rail. Yeah I might just get the inline adjustable prop valve hah. I need to check which of my brake lines are just a bit too short and hopefully it's the rear circuit, because the inline prop valve will give a bit more clearance. Nothing stressed or taut or anything like that, it's just that the line was shorter than I'd have liked because it was a 6 metre roll rather than the usual 7 m/25 ft roll.
No lock-up problems that I can test because the car's still on stands, but once everything's back together (still waiting on firm feel and my steering box lol) I'll do test runs with the current 15/16 cylinders and see what adjustments are needed.
I've seen some older posts here where someone converted to front discs but never installed the metering valve and all it did was make the car nosedive during hard stops, but not affect wheel lockup.
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