Back brake is very poor-new pads ,enough correct fluid etc BUT foreward controls fitted (POORLY)before i got it,- lots of play in linkage rods etc How can i determine if problem actually is in linkage.

disc brakes can only lock up because the fluid is not allowed to return to the master cylinder reservoir
That is normally because the compensating port in the master cylinder is blocked
You say that you have fitted 2 new master cylinders but did you check the brake pedal free play because if there is not enough free play then the piston cup covers the compensating hole and the problem that you have is the result

Do you mean it is shifting poorly/slipping, or are you wanting harder shifts than stock? If shifting poorly, when did you last replace trans fluid and filter?

Jack the car corner your working on, remove the wheel/tire, use a c-clamp to squeeze the brake caliper and outside pad together pushing all the fluid back to master cylinder, remove the 2 slide bolts attaching the caliper to the bracket, slide the caliper off, replace the pads, pit the caliper back on and tighten the 2 slide bolts, now pump the brake pedal to get the fluid back into the caliper, put the wheel/tire back on and back on the ground. Same with other side. Your only problem is finding the c-clamp you need that's just large enough to squeeze the piston back in to begin with. Place the c-frame on the back of the caliper and the screw in part on the outer brake pad and turn it in till it stops. Gives you lots of room for removal and new pads will fit right in.

The new pads may require a few kilometers of riding before they bed into the new disks.
Failing this, try reverse bleeding of your brakes. Use a large syringe and tube. Fill the syringe with brake fluid, being careful to remove all air from the syringe and tube. Connect to bleed nipple, and with the brake lever fluid reservoir cap removed, compress the brake fluid back up to the reservoir. Your syringe will need to be large enough to contain more fluid than caliper and line combined for this procedure.

I want to say the 1st tech is correct. But i also have an idea from a different point of view. Here we go, #1: depending on carb type and type of pump...could be dirty and the pistons can not get a good seal. If the cylinder hole is worn out, or the pistons seals are old shelf parts and can not contact the cylinder wall corectly and evenly. Poor linkage to rod geometry also has play here as was stated by the 1st techs answer. #2: If it is linkage and diaphragm type then same thing about old shelf parts...basically maybe your new parts are "bad" new parts...or you got worn out linkages, or poor linkage to rod geometry...or worn out carbs.
What to do to fix? Boar and sleeve if cylinder type...specialty carb place if one still exists, replace all 3 carbs, convert to 2 4bbl carbs or 1 large carb untill you can finds what you want. Convert to fuel injection...lots of very resonable stuff out there these days...make great power even self tuning stuff...please see "youtube" and reality tv shows...browse a few chevy periodicals (all are online) chevy *********, super chevy, car craft, hotrod, Summit racing, rock auto (yes Idid just say that) swap meets, car club sales...good luck

It is not uncommon for discs including car discs after changing pads for the disc to squeel.

Because the disc will inevitably wear with use, the old brake pad that causes the wear will also be worn to fit perfectly to the wear on the disc as at engages, whereby grooves in the disc that are formed will be identically paired to that on the brake pad.

Squeeling is also common for old pads, as well as poor quality badly manufactured cheap pads, or excessive use of brake pads - where they get burnt and are too hard against the disc or dust fibres from the brake pad get caught within pad and disc. It could even be a combination of these that causes squeeling.

When you change the pads which are now new and being smooth and unworn they do not mate up anymore with the groove marks created from the wear of the old pads. The discs when brake pressure is applied will always vibrate to some extent, even on new bikes, however if imperfections exist such as the fact there is no wear marks from new pads rubbing against the old disc with its worn imperfections or even improper fitting etc occurs - they will sing in a high pitched squeel at certain speeds, even with good quality pads.

Other than ensuring all items are set up correctly, get a good quality brake pad and if it continues squeeling consider replacing the disc rotar itself or get the old one machined by a mechanic shop to be smooth again.

open rear brake resover cap ,drain some fluid out,remove brake pads,clean piston,push piston back into caliper,refit pads,pump up rear brake

Loosen the brake fluid reservoir cap to allow pressure to escape. Use a large C clamp connecting it to the front of the piston and back of the housing. Turn the screw on the clamp to compress the brake piston then release the clamp. The piston will remain compressed allowing you the fit the housing over the new brake pads.

Diagram is unnecessry.
Pull both front wheels
Work one at a time leaving the other for reference
Find the retention spring ... its a paperclip thickness wire spring... dislodge center retention loop and undo from both ends which engage brake pad retention rods.
Pull the rods out
Pull the old pads out... laying everything on the ground
Use scrap wooden wedges to pry between the rotor and the caliper ...inside and out until the calipers are fully withdrawn. Use the new pads to measure if they are far enough back.
Note... the brake fluid will have dropped in the master cylinder over many months of wear... do not fill it. When the calipers are pushed back they will fill the reservoir again... one wheel at a time.
Be sure to identify the correct pad based on what you took out and what is on the other side (if you screwed up the layout)
Make sure to take the old antisqueek backing plate from the old pad and place it on the new pad. They don't use lubricant on these anymore.
Replace the pads... the retention rods... then the retention spring
Do the other side
Then progressively step on the brake peddle until full tension is felt
Check the master cylinder reservoir... probably needs no additional fluid.
New pads will register in existing ridges in each disc in no time at all.
You should check the discs to see if they need to be replaced during this operation...using some calipers