If you are not a member of Cardone's ProTech , you should be. I was going through some of their technical bulletins, and I stumbled on this one about phenolic pistons.
When I say the word "Phenolic" around some older technicians, it is like exposing Superman to Kryptonite. But, their view is based off myth more than science.

This technical bulletin from Cardone proves that phenolic pistons are required for some applications: 

Brake Fade, Excessive Pedal Travel:

Application: Ford E250, E350, F250 F350 1991-89
Problem: Excessive brake pedal travel or brake pedal fade may occur during application, when heavily loaded or on steep downhill grades.

Cause: This maybe caused by the original brake caliper piston material.

Solution: According to Ford TSB 91-20-8, changing to phenolic piston equipped calipers may correct problem.  Applies to 1991-89 E-250, E-350, F-250, F-350 trucks built prior to 5/1/91.

Notes: Before replacing calipers be sure to perform brake system tests that eliminate other possible vehicle problems that might cause the fade or excessive pedal travel. Refer to the Ford Light Truck Shop Manual, Section 12-01, for diagnostic procedures and adjustments that may be necessary.  If adjustment does not correct concern, replace both calipers with phenolic piston equipped remanufactured units.  Refer to CARDONE catalog for correct part numbers. 
CARDONE lo...