A very common cause on these is a swollen brake hose...they deteriorate inside and swell up, causing the caliper to hold pressure and not able to return after pedal application..very common w/aged hoses.. pedal pressure is high enough to over-come the restriction and apply force to caliper , but return/release pressure is residual, so the hose acts as a one-way valve due to the interior swell/restriction....

Here is a simple diagnostics trick to iso the problem:

Jack the suspect wheel off ground and apply the brake pedal as hard as you can...now get out and try to hand turn the wheel... if NO , open that bleeder..if the wheel then turns freely, you have a swollen hose...if it does not, you have a frozen caliper [ or e -brake bind]

Simple.............opening the bleeder will release any line pressure to the caliper, so you then know the hose is shot...but if you release pressure w/open bleeder and the caliper is stll holding that wheel, it is not line pressure that is the cause b/c there is no line pressure w/bleeder open.............

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A Dalton