[wolfman]

I have just managed to get a disc brake set up for my 71 chally, It is from a 73 challenger and includes stub axles,rotors backplates and callipers. My question is why do I need to change my M/cylinder? I have just fitted a new power brake one as my old one was leaking and it was an oddball one to get hold of too. The info I have found has been a bit confusing as to which one is correct for the application the bore diameter on my new one is 1" if I remember correctly any advice guys would be a great help.

[fbernard]

If you have the OEM brakes from the '73, bore diameter should not be the issue (the OEM calipers don't have huge brake fluid volume requirements for one pump stroke). Bore diameter would be something to look at for 3-, 4-, 6- or 8-piston calipers.

The issue is with the front brake fluid reservoir size in the master cylinder. Discs need volume (to account for pad wear).
On disc/drum MCs you have a large reservoir (for the discs) and a smaller one (for the drums). You can see that simply by looking at the shape of the MC cover plate.

Another issue is with the residual pressure valves that are integrated into the [drum] brake fluid outlet on all MCs.
Residual pressure is beneficial on drum brakes because it means the wheel cylinders won't retract too much when you release the brakes, thus giving you braking power quicker next time you depress the pedal.
You do NOT want a residual valve on the disc circuit, for it will keep the brake slightly tightened all the time, and that leads to disc and pad glazing, fluid boiling, etc...

Actually, there could be a 2lb residual valve on the disc circuit . Drum circuits use a 10lb valve. That valve is not very necessary though, even on the rear, since modern wheel cylinders also incorporate the valve.

You will also need a proportioning valve on the rear (drum) circuit.

[dave-r]

An adjustable rear proportioning valve is best.

[wolfman]

If you have the OEM brakes from the '73, bore diameter should not be the issue (the OEM calipers don't have huge brake fluid volume requirements for one pump stroke). Bore diameter would be something to look at for 3-, 4-, 6- or 8-piston calipers.

The issue is with the front brake fluid reservoir size in the master cylinder. Discs need volume (to account for pad wear).
On disc/drum MCs you have a large reservoir (for the discs) and a smaller one (for the drums). You can see that simply by looking at the shape of the MC cover plate.

Another issue is with the residual pressure valves that are integrated into the [drum] brake fluid outlet on all MCs.
Residual pressure is beneficial on drum brakes because it means the wheel cylinders won't retract too much when you release the brakes, thus giving you braking power quicker next time you depress the pedal.
You do NOT want a residual valve on the disc circuit, for it will keep the brake slightly tightened all the time, and that leads to disc and pad glazing, fluid boiling, etc...

Actually, there could be a 2lb residual valve on the disc circuit . Drum circuits use a 10lb valve. That valve is not very necessary though, even on the rear, since modern wheel cylinders also incorporate the valve.

You will also need a proportioning valve on the rear (drum) circuit.

The set up is oem and I think I was lucky to nab it first. So I could pull the valve on the front system and retain it on the rears and see how it goes. Or should I remove both if I fit a proportioning valve on the rear,any recomendations on the best place to get one.

[Goldenblack440]

Bernard, that was a great answer -very informative and i'd like to be the first to thank you for it. I have copied it into my General info (brakes) -hope you don't mind !

I too have found a complete power disk brake setup from a '70 383 Cuda to replace the unboosted front drums on my 71. But after installing a new bolt in alum/plastic MCyl in my '73 (see pic) i think i will use the original OEM Mcyl. That new one leaks constantly from the twin plastic caps (made in china!!) and nothing i can do has stopped that. Also the aluminium body, although made in USA is lighter but the whole affair feels flimsy. And just doesn't look as good

[wolfman]

Bernard, that was a great answer -very informative and i'd like to be the first to thank you for it. I have copied it into my General info (brakes) -hope you don't mind !

I too have found a complete power disk brake setup from a '70 383 Cuda to replace the unboosted front drums on my 71. But after installing a new bolt in alum/plastic MCyl in my '73 (see pic) i think i will use the original OEM Mcyl. That new one leaks constantly from the twin plastic caps (made in china!!) and nothing i can do has stopped that. Also the aluminium body, although made in USA is lighter but the whole affair feels flimsy. And just doesn't look as good

I have to agree the first replacement I was offered was a chinese job and was very badly made.I then found an american built one that is far better,I love this site for the great information freely offered by everyone too.It is a shame I cannot often give words of advice thou,you all know more than me

[fbernard]

[ So I could pull the valve on the front system and retain it on the rears and see how it goes. Or should I remove both if I fit a proportioning valve on the rear,any recomendations on the best place to get one.

The residual pressure valves and the proportioning valve are two different matters. Totally unrelated.

The proportioning valve (and, as Dave said, an adjustable kind is the only way to go - OEM valves were the same part number for Darts and C-Bodies!) will enable you to limit the pressure transmitted to the rear brakes.
What this means is that it will help you avoid the lightning-fast 180° turn when you hit the brakes in an emergency under the rain.

Once you have fitted an adjustable proportioning valve, find a nice empty place qhere you can do emergency stops without hitting anything, impeding traffic or getting a ticket.
Proceed to do emergency stops from 40+ mph.
If the rear wheels lock up before the front ones, add a few turns to the proportiong valve screw (screwing in).

If it's not easy to guess which wheels lock first, you may need a friend standing by and looking at your car from the outside.

Your aim is to set it so the front wheels lock up just before the rears do.

Ideally, you could do that right after some rain, since a wet ground is the worst case (or do it on dry ground and add a few turns - I did it, I hate rain).

Once this is done properly, even under heavy rain, if you mash the brakes (or if you install a monster hydraulic brake booster like I did), the car will skid but stay straight. You will still need fresh pants , but no bodywork.

[wolfman]

[ So I could pull the valve on the front system and retain it on the rears and see how it goes. Or should I remove both if I fit a proportioning valve on the rear,any recomendations on the best place to get one.

The residual pressure valves and the proportioning valve are two different matters. Totally unrelated.

The proportioning valve (and, as Dave said, an adjustable kind is the only way to go - OEM valves were the same part number for Darts and C-Bodies!) will enable you to limit the pressure transmitted to the rear brakes.
What this means is that it will help you avoid the lightning-fast 180° turn when you hit the brakes in an emergency under the rain.

Once you have fitted an adjustable proportioning valve, find a nice empty place qhere you can do emergency stops without hitting anything, impeding traffic or getting a ticket.
Proceed to do emergency stops from 40+ mph.
If the rear wheels lock up before the front ones, add a few turns to the proportiong valve screw (screwing in).

If it's not easy to guess which wheels lock first, you may need a friend standing by and looking at your car from the outside.

Your aim is to set it so the front wheels lock up just before the rears do.

Ideally, you could do that right after some rain, since a wet ground is the worst case (or do it on dry ground and add a few turns - I did it, I hate rain).

Once this is done properly, even under heavy rain, if you mash the brakes (or if you install a monster hydraulic brake booster like I did), the car will skid but stay straight. You will still need fresh pants , but no bodywork.

Thank you for that information, my response may have been confusing I intend to fit a adjustable valve to the backs as advised. should I remove the residual valve from the m/c on the front brakes to avoid any form of hydraulic lock developing.If I had fitted the adj valve before I could have tested it in all the snow we have now.

[fbernard]

[ should I remove the residual valve from the m/c on the front brakes to avoid any form of hydraulic lock developing.If I had fitted the adj valve before I could have tested it in all the snow we have now.

Yes, you should remove it.

Snow?? If my memory is correct (I drove through 2 inches of snow this morning to get to work ), that's the only circumstance for which (rear) wheel lock is actually desireable...

[wolfman]

[ should I remove the residual valve from the m/c on the front brakes to avoid any form of hydraulic lock developing.If I had fitted the adj valve before I could have tested it in all the snow we have now.

Yes, you should remove it.

Snow?? If my memory is correct (I drove through 2 inches of snow this morning to get to work ), that's the only circumstance for which (rear) wheel lock is actually desireable...

I know I love driving in the snow,It is rare we have it round here so there are so many idiot drivers around here you take your life in your hands every time you go out. My daughter has just last month past her driving test so this has been a great chance to take her out to learn the best way to drive in these conditions. She loved doing 180 degree handbrake turns