Im replumbing the brakes on my 51 DeDoto....

Im using a 91-96 Roadmaster master cyl and combination valve.(same as ImpalaSS with rear disc)

Front brakes are 1978 Fury LateB disc ...rears are Ford Explorer disc

The line from the master to the combo valve is larger for the rear brakes. From the modfied combo valve the rear goes to a Wilwood adjustable prop valve and the front goes to a line lock. Both the prop valve and line lock use 1/8" 27 npt fittings .

Can I use 1/4" lines from the valves out? Does the rear have to be 3/16"?

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Yeah....Im the one who destroyed this rare, vintage automobile.....

So long as you can get/make the correct inverted double flare or metric bubble flare as required for your fittings, line nuts, & hoses- then yes. Just make sure that the combi valve stuff IS 1/4". I've had a couple GM's that had 6mm line going to the rear brakes (1/4" wouldn't work).

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63 Valiant Wagon
225 - 4 bbl

The difference in brake tube "swell" at max pressure between 1/4" and 3/16" is surprisingly large. I did the math once and was very surprised at the results. Enough surprised that I had another ME check my work and he too was surprised. That "swell" is a difference in volume, which will make the pedal feel 'mushy' to the driver.
None of the aftermarket brake mfg's consider 1/4" viable for brakes, clutches only. That said, I know of several examples of 1/4" used by the OE's. I wouldn't do it or suggest it, go 3/16" if at all possible.

The reason for 1/8" NPT is that fittings in 1/16" NPT aren't terribly common. Can be had for sure, but aren't common.

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Thom

Cross-threaded is tighter than Lock-tite

Thankyou gentlemen.....We did everything in 3/16 from the combo valve out.

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Yeah....Im the one who destroyed this rare, vintage automobile.....