have you got the old one out? sometimes they are siezed so badly they won't budge and end up needing to be drilled out. that can be risky if you inadvertently drill too deep and damage the seat in the wheel cylinder. you can get backwards drill bits that may help you here because they turn backwards so if there is a chance the bleeder may start to turn the drill will start that motion as you work. if you have the old one out, one of the old ones out, you could take it for a match up. if you don't have it out yet then I would say pick up a few sizes and have them ready for when you try to remove the old bleeder. also pick up a new wheel cylinder for the appropriate side of the truck. if they have a new wheel cylinder in stock you may be able to get the correct sized bleeder from that as a sample. anyway, with a handfull of bleeder sizes, pick the bleeder that fits and take the other stuff back when done. if the bleeder is siezed beyond getting it out then you have the new wheel cylinder ready. check the brake lines as well because they may not wanna come out of the wheel cylinder either. in that case it is a good bet that all the lines need to be replaced for safety sake because they have rusted and are thinner walled in some spots. this could cause a rupture at the least convenient moment, like when somebody stops short in front of you. since the old brake systems are a single system, meaning the front and rear brakes run off the same circuit, then you would lose ALL your brakes at the same time. another good reason to update the master cylinder to a dual system master cylinder that separates the front and rear circuits for safety sake. I always advise guys to check the whole system from front to rear starting at the pedal and working down. check/repair the brake linkage from the pedal down to the master and ensure all the linkages and bump stops are in good working order, not rubbing on anything, adjusted correctly and the return spring is also in place. lube the pivot points, make sure the pedal pad is in good shape. check the master cylinder for leakage-pull back the rubber boot on the plunger and check, look in the reservoir and check for dirty fluid and deposits in the reservoir, a proper sealing and venting cap, brown fluid means it has been overheated or it has absorbed condensation. brake fluid likes condensation and absorbs it but that also turns the color of the fluid brown. this is why it is recommended to bleed the fluid yearly or at least at every brake job. more humid climates are harder on the brake fluid for the same reason. then look at the steel lines from front to rear especially under clamps and around connections where they tend to hold water, mud etc and end up with rust and get thin. then check the hoses for soft spots, bulges, cracking etc-bend the hoses and look for cracks in the rubber and around the crimped ends. next pull the drums off and check the wheel cylinders for leakage. you need to pull back the dust caps to see in there. a dry on the outside rubber boot doesn't mean the cylinder is not seeping. move the brake shoes so that the wheel cylinders also need to move the pistons to ensure they are not getting gummed up. check the adjusters and other related hardware as well as the remaining lining compared to the service limit. also, check the backing plates in the contact areas where the shoes rub. sometimes the shoes have worn a groove into the backing plate and that means the shoes can get hung up in that groove and not allow the brakes to be adjusted properly or operate properly. check the drums for ridges and service limit as well as the hub/axle seals. check the park brake for proper operation, proper adjuster operation, easy cable movement in the sheaves and for full return when released. check the brake lights to ensure they are operational. adjust the brakes when done. this needs to be done with the park brake backed off, adjust the brakes, then adjust the park brake. then bleed the whole system one wheel at a time until fresh clean fluid appears at each wheel-with no bubbles. start at the wheel that is furthest away from the master-usually the right rear, then left rear, then right front and finally left front. after that you should have no problem with the brakes for awhile.
as a safety hint, keep a hand sprayer of water close by so when you are removing the drums you can spray down the brakes and keep the airborne contaminants out of your lungs. the stuff is killer. also, brake fluid is harsh on the skin so a pair of rubber/nitrile gloves is a bonus as well as some safety glasses. parts are cheap but you only get one set of eyeballs.
here is a link to the master cylinder conversion
http://67-72chevytrucks.com/vboard/s...d.php?t=617103
a brothers video showing the process of updating the master cylinder to a dual system
https://www.youtube.com/watch?v=_eyhUseqNq8
a supplier with pics and prices-quick search. I just picked the first site that came up.
https://www.performanceonline.com/19...ders-and-kits/
hope that helps