Tom,

Were the caliper's new or rebuilt?

I've seen the pistons hang up on some poorly rebuilt calipers, simply
because they didn't replace the piston, or the bore in which the piston
slides in and out of is galled, or pitted so bad that it hangs up, which
the rebuilder didn't take care of.
Or the caliper got some garbage in between the piston and the bore,
which would cause it to hang up, durring the 16 months since you changed
them.

It could also be the master isn't letting the fluid return, but only on
one side is pretty improbable.

The proportioning valve regulates the pressure from front to rear, not
side to side, so I don't think this is it either.

Check out the rubber hoses that connect the caliper's to the hard line.
If they were collapsed internally they could cause you some problems, on
the return of the fluid.

Pressure bleed the front brakes and this will tell you if there is any
restrictions in the brake line.

It's a process of elimination situation---

I'd start with the caliper and work your way up.

Later,

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Terry--
74 Campmobile- (Clementine)
85 GL- (Delilah)
86 BMW 325 ES- (Eva)