if you were local Id say tow it over and we could go thru it, isolate the cause and correct the problem
but as your not located in my local area
Id suggest you read the thread, links and sub links I'll post below, carefully.
step one is BLEED all four brake wheel cylinders and carefully inspect them,the brake pads,and then inspect the master cylinder, Id suspect that that rear wheel cylinder leak is allowing air into the system, but there is also very likely moisture in the old brake fluid, so replacing all the brake fluid by bleeding all the brake lines makes sense as a first step, then id suggest replacing or rebuilding the wheel cylinder as that is more than likely the cause , much more likely than the master cylinder and THAT wheel cylinder must be repaired or replaced in any case. before further testing
we had to replace both rear wheel brake cylinders on my brother-in-laws 1974 big block corvette before the brakes functioned correctly and both only showed minor leaks yet even those minor leaks had over time forced us to also replace the brake pads and calipers which were both worn
bleeding brakes is generally done like this , picture below depicts, with one person pumping the brake petal, while a second person opens the bleed valve, as pressure is applied, and the second person opens the bleed valve to let air and oil fluid bleed out down into the container while the person opening and closing the bleeder valve watches the clear tube to see its contents until clear new air free fluid emerges from the bleed valve several times in succession. the guy at the bleed valve closes it before the petal is allowed to return to it fully up location,and opens it as pressures applied, generally to a voice command of " push down, hold, release", " push down, hold, release", " push down, hold, release", " push down, hold, release", until the fluids clear and air free
theres a second faster way,
a vacuum hand pump like the one pictured below can be located at harbor freight or most large auto parts stores, a clear section of hose can be connected to the medical container pictured below with only two vents connecting to clear vynil tube (usually 5/16") from the brake bleeder to the container and the second medical container connection with a second hose to the brake caliper or slave cylinder brake fluid bleeder valve, you simply open the valve and use the vacuum pump to lower the container vacuum, the brake fluid drains rather rapidly into the container thru the open path, as you pump the brakes a few times, so you,ll want to locate the medical container where you can watch the master cylinder brake fluid level if your doing it by yourself,or more likely have a friend watch and fill it as you drain the contents of oil brake fluid so they refill it as required with new fluid.
http://www.mityvac.com/pages/products_hvp.asp
http://www.aaawholesalecompany.com/bem-484410-pk.html
NOTICE THEY COME singly for about $8 each or 12 to a carton and cost about $60 a dozen so get two to 6 buddies to split the cost
medical supplys can be useful bleeding brakes
http://www.corvettemagazine.com/tech-ar ... ke-system/
http://www.vetteweb.com/tech/vemp_0310_ ... _overhaul/
http://www.popularmechanics.com/cars/ho ... ce/4213448
viewtopic.php?f=34&t=380&p=19110&hilit=brakes+trouble#p19110
AS USUAL READING LINKS AND SUB LINKS MAY HELP
http://www.corvettemagazine.com/tech-articles/1968-1973-corvette-power-brake-conversion/
http://www.fullthrottlecorvette.com/1968-Corvette-Power-Brake-Conversion-Kit_p_1951.html
https://www.autozone.com/brakes-and-traction-control/brake-power-booster
http://www.zip-corvette.com/68-82-c3/brakes/corvette-power-brake-booster.html
http://www.corvettepacifica.com/categories/59/brake-kits
http://www.cssbinc.com/corvette-brake-kit.aspx
http://www.corvettemagazine.com/tech-articles/1965-1982-corvette-brake-rotor-replacement/
http://www.corvettemagazine.com/tech-articles/bleeding-your-1967-1982-corvette-brake-system/
check rock auto
http://www.rockauto.com/en/catalog/...040870,brake+&+wheel+hub,master+cylinder,1836
http://www.rockauto.com/en/catalog/...70,brake+&+wheel+hub,power+brake+booster,1884
http://www.rockauto.com/en/catalog/chevrolet,1968,corvette,5.3l+327cid+v8,1040870,brake+&+wheel+hub,rotor+&+brake+pad+kit,13824
HAVING A SHOP MANUAL HELPS
JOBS LIKE THIS ARE BEST ACCOMPLISHED on a solid flat concrete floor with 4 12 ton rated jack stands
http://www.harborfreight.com/12-ton-jac ... 34924.html
viewtopic.php?f=34&t=9536&p=39159&hilit=1974#p39159
viewtopic.php?f=34&t=9917&p=37986&hilit=1974#p37986
