Manual Disc/Drum Master Cylinder??

['70_402]

I'm converting my 68 to disc up front, but keeping the drums in the rear. i currently have manual drums all around... Do i need to swap my master cylinder out?? If so, just a 71/72 master work?? do i need to get a proportion valve or is it built into the master??? I'm not carving corners or running autocross here... I just wanna stop.

[demian5]

Is it manual or power brakes?

Yes a 1972 master and combo valve will work. Just bolt in and go.

['70_402]

Staying with Manual brakes for now.

[RichardJ]

The use of a Proportion Valve has nothing to do with power or manual brakes.

With a drum/drum system, you are delivering the same pressure to the rear wheel cylinders as is delivered to the front wheel cylinders.
If you are pushing on the pedal hard enough to deliver 400 psi to the front wheel cylinders, you are also delivering 400 psi to the rear cylinders. 1200 psi to the front, 1200psi to the rear.


If you replace the front drums with discs and don't add a proportion valve to the rear circuit, you will be providing too much pressure to the rear, at higher pressures.
At 400 psi front and rear, the brakes will work fine. At higher pressures, the rear drum brakes will lock-up at much lower pressures than the front disc calipers will lock.

A proportion valve reduces the rear pressure at a proportional rate, but only above a threshold pressure point.
In the example below, the Valve allows the same pressure, front and rear below 600psi. At higher pressures, say 1200 psi, the valve reduces the rear pressure to 750psi.

It works the same with manual or with power booster.
A proportion valve would still be required and work the same with a dual master cylinder or with an older Single master cylinder.

[angelobackwards]

I’m looking to do a disc upgrade as well. Planning on keeping manual brakes as well. Are the master cylinders off power and manual drum brakes the same as long as I get a disc/drum master and proportioning valve?

[michael bustamante]

were manual disc brakes even an option in 71-72?

[angelobackwards]

Hey sorry forgot to mention mine is a 64.

[68Gold/white]

Manual discs were an option in 71 and 72.

Get a 71 or 72 manual brake master cylinder, and the 71 or 72 proportioning valve to go with it. Do NOT use a drum brake m/cyl.

I have both a 67 and 68 C10, plan to go with manual disc on both. Only thing I am not sure on is rod from brake pedal to m/cyl. I did not get those when I got my salvage parts....

[jocko]

70 402, what 68Gold/White said is good advice. You indeed want a 71-72 "manual brake" mc. Best way to identify is that the hole in the piston that your brake rod lever fists into is deep - just like the one you'll be removing. A "power" 71-72 mc would have a shallow indent in the mc's piston. I'd use a standard distribution block/combo valve from a 71/72. It will provide the same level of notification and circuit isolation your 68's current mc does. On the other hand, if you want to be able to "tune" the front-to-rear bias once it's all installed, an aftermarket proportioning valve would be an option also.

Btw, is your 68 a C10 or C20? Make sure you specify when ordering, the mc's are different.

[jocko]

Originally Posted by angelobackwards

I’m looking to do a disc upgrade as well. Planning on keeping manual brakes as well. Are the master cylinders off power and manual drum brakes the same as long as I get a disc/drum master and proportioning valve?

Angelo, you'll be doing a somewhat more extensive conversion since the 64 has a single-reservoir mc. You'll need a "manual" disk/drum mc (just like one the op will get, say from a 71-72 dual reservoir with manual brakes), and you'll also need to plumb new lines and re-wicker the old ones a bit (or just plumb ALL new lines (that's what I recommend)). Generally speaking, your existing lines can provide the basis for your rear brakes because they are 1/4" and the rear brakes should remain 1/4" tubing. If you re-use them, you'll need to disconnect and block off the port at the T where the line splits and goes to the current front wheel cylinders. That spot is usually just behind the front crossmember. Once you cap what used to be the front brake port of the 3-port T, you can just connect the mc end of the original line coming from the mc to your rear brake exit port of the proportioning valve or distribution block you will use - then will need a new short line from the valve/block to the rear brake circuit port of the mc. When I say "rear" brake circuit port - I mean the one that is FOR the rear brakes, not the one at the rear physically, necessarily. If you are using a C10 mc, the front port is for the front brakes (and should be 3/16" line) and the rear port is for the rear brakes (and should be 1/4" line) - i.e. it makes sense - front to front, rear to rear (thru your valve or block). If you are using a C20 mc, it's the opposite - front reservoir (and port) is for the rear drum brakes, and rear reservoir/port is for the front brakes. Disks generally use 3/16" lines and drums use 1/4". Then, you'd still need to completely plumb (or buy pre-bent) front brake lines. The master cylinders for manual brakes are different from the ones for power brakes (see post above) - so make sure you specify which you need.

Here is a thread about a 66 dual res conversion (not a disk brake conversion, it's all drum, so there's no need for a distrib block/prop valve, or for any 3/16" lines). Might provide some helpful pics. http://67-72chevytrucks.com/vboard/s...d.php?t=487384

Also keep in mind the decisions you'll need to make before pitching in - i.e. 5 vs 6 lug, etc. If you think you'll ever want to go with power brakes, save money in the long run and do it all now. CaptainFab sells a great pb booster bracket that is snug to the firewall and uses stock gm boosters. Just a thought. If you want to stay 6 lug, you'll likely need aftermarket rotors. Also keep in mind that stock 64 6 lug steelies will not normally clear the caliper in front - 71 or newer K5/K10 wheels are what you really need to stay with stock steelies. ID them by having 4 cap nubs rather than 3. If you want to switch to 5 lug, there are kits available, and it can provide more wheel style choices. But keep in mind you then will also probably want to swap to 5 lug rear axles so you only need 1 spare tire...Kits are available for that also and aren't that expensive.

[Stephen717]

Sorry to revive this thread but I am having trouble finding a 71/72 manual mc for my truck, I'm swapping the body onto a 1972 frame that came with discs up front and have upgraded the braking system so it has wilwood dual piston calipers up front (yes I know that these are overkill for my truck but I got a good deal on them) and 14 bolt drums in the rear, I have everything fitting nice enough to roll the frame around the shop and I'm about to start swapping the body over but I need to upgrade from the drum mc that came on the 69. I would like to stay with manual brakes but like I said I'm having trouble finding the manual mc for my truck. It's a c20 frame. Does anyone know where I can find one?

[68Gold/white]

Originally Posted by Stephen717

Sorry to revive this thread but I am having trouble finding a 71/72 manual mc for my truck, I'm swapping the body onto a 1972 frame that came with discs up front and have upgraded the braking system so it has wilwood dual piston calipers up front (yes I know that these are overkill for my truck but I got a good deal on them) and 14 bolt drums in the rear, I have everything fitting nice enough to roll the frame around the shop and I'm about to start swapping the body over but I need to upgrade from the drum mc that came on the 69. I would like to stay with manual brakes but like I said I'm having trouble finding the manual mc for my truck. It's a c20 frame. Does anyone know where I can find one?

Who have you called??? A parts store should have one available...

[jocko]

Originally Posted by Stephen717

Sorry to revive this thread but I am having trouble finding a 71/72 manual mc for my truck, I'm swapping the body onto a 1972 frame that came with discs up front and have upgraded the braking system so it has wilwood dual piston calipers up front (yes I know that these are overkill for my truck but I got a good deal on them) and 14 bolt drums in the rear, I have everything fitting nice enough to roll the frame around the shop and I'm about to start swapping the body over but I need to upgrade from the drum mc that came on the 69. I would like to stay with manual brakes but like I said I'm having trouble finding the manual mc for my truck. It's a c20 frame. Does anyone know where I can find one?

I believe power brakes became standard in 71/72 for 3/4 ton and above (remained optional on 1/2 tons) - this could be why you can't find a manual MC in that year range - if you're searching for a C20 MC. And note that the C10 and C20 MCs are indeed different in 71/72. I'd sort out a way to add the booster, you won't regret it. May I ask what the hesitance to adding a booster is? Aesthetics for a custom build or something like that? One comment - since you've upgraded to a very capable front disk system and have drums on the back - I think an adjustable prop valve, though not a stock item on a 71/72 disk/drum system, would be a good idea in your application. Would also like to ask about your brake lines - have you updated the front lines so that you have 1/4 to rear drums (still) and new 3/16 lines to front disks? While the original 1/4 all the way around from the previous all drum system will "work" - it's not optimized for a disk system up front (at least not a stock one).

[Stephen717]

I would have to look at the brake line kit I got to know which size it is, I know it is for a 72 c20 but I'm not sure if it's all the same size line or not as it's been awhile since I looked at the lines since putting them on. I would like to add a proportioning valve and don't mind if it doesn't look correct. And the reason I'm hesitant is because I am planning on keeping the 250 in my truck for the time being and upgrading to something with more power later on down the road, but I don't know if the 250 would have sufficient enough vacuum to run a power brake booster, and I don't want to run any kind of hydroboost system

[Steeveedee]

Unless you've done something crazy with a cam or carburetion, the 250 will pull enough vacuum for power brakes. One thing to consider though, is if the engine quits, you have no power steering and the booster will take about 3 pumps until you have no assist. the fuel pump quit on my Astrovan many years ago, and my wife barely managed to steer it off the freeway and get it stopped. If she'd been in bad traffic, it could have been ugly.

[Stephen717]

Ive had that problem with brakes and steering before which is why I don't want a hydroboost system, and I am going to put a Chinese Weber 32/36 dgv 2 barrel on it, and I'm going to port and polish the head. Other than that I'm leaving the motor alone. I'd be willing to try power brakes if y'all think I would have enough vacuum pressure to power it but I would like to stay with manual brakes for the simplicity of it if I can

[Steeveedee]

The only difference between the power and manual brakes is the (not much) higher pedal effort. It's not as high an effort as if you have power brakes and the engine quits. You should be fine either way you go. I don't think that the carb you are putting on is going to be a problem. It looks like it is progressive linkage, so won't be too much carburetor. How many CFM is it, BTW?

[Stephen717]

I think it's somewhere around 350-400 cfm. I'd have to double check on that though.

[Steeveedee]

Originally Posted by Stephen717

I think it's somewhere around 350-400 cfm. I'd have to double check on that though.

Since it has a progressive linkage (primary and secondary throttle bores) you should be just fine with it, and maybe pick up a little performance.

[Stephen717]

Awesome, also I checked the brake lines on the frame and I do have the 3/16" lines up front and 1/4" for the rear so I should just need the prop valve, the brake booster/master cylinder, the rear brake shoes, and the emergency brake cables to finish everything off. Thanks for all the help