Brake line question...troubles

[verona33]

My master cylinder on my 86 the front port is larger than the rearboth in line size and thread size. However, when it coils then to the prob valve, the larger line is the same on the prop valve but the fitting is smaller. The fitting itself is the same size as the rear smaller line on mast cylinder and prop valve. What im asking, do they make single fittings to adapt down to the small thread in the proportioming valve while still accepting the larger line? When i get some money ill become a contributor i caught some flak over that.
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[ray_mcavoy]

As you've noticed, GM used inverted flare nuts with larger than standard thread sizes in certain locations like the master cylinder & combination valve in order to prevent the lines from being incorrectly installed. You can buy these special oversize nuts for both the 3/16" and 1/4" lines but you'll need to verify the thread size to make sure you get the right ones. Here are a few examples:

1/2"-20 thread for 3/16" line --> http://www.thetoolwarehouse.net/p-6411-sur-r-br150.aspx
9/16"-18 thread for 3/16" line --> http://www.thetoolwarehouse.net/p-6412-sur-r-br155.aspx

1/2"-20 thread for 1/4" line --> http://www.thetoolwarehouse.net/p-64...-r-br1650.aspx
9/16"-18 thread for 1/4" line --> http://www.thetoolwarehouse.net/p-64...-r-br1600.aspx

Weatherhead and other companies also make inverted flare adapters that can be used to adapt from the standard nut size to the special oversize thread sizes in master cylinders and combination valves. These adapters come in handy if you're using sections of pre-flared brake line and you don't want to cut & re-flare them to change out the nuts.

[verona33]

Thanks for the info. So basically its not a matter of downsizeing the proportioning valve nut because its typical...the master cylinder has juat been upsized, not the other way around, correct? So getting that fitting for the prop valve will be no prob? Thank you for helping.
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[ray_mcavoy]

Yes, both of the master cylinder nuts are special oversize ones. The combination valve uses a mixture of standard and oversize nuts on the various inlets/outlets. Most of the ones I've dealt with have used the following sizes:

- Master cylinder front brake outlet (to combo valve) = 3/16" line with 1/2"-20 thread nut. Note that the front brake outlet is actually the rear port (closest the firewall) on some master cylinders.
- Combo valve front brake inlet (from master cyl) = 3/16" line with 1/2"-20 thread nut (same as the master cylinder front outlet).
- Combo valve front brake outlets (both of them to front brakes) = 3/16" line with standard size 3/8"-24 nuts.

- Master cylinder rear brake outlet (to combo valve) = 1/4" line with 9/16"-18 thread nut.
- Combo valve rear brake inlet (from master cyl) = 1/4" line with standard size 7/16"-24 thread nut.
- Combo valve rear brake outlet (to rear brakes) = 1/4" line with 9/16"-18 thread nut (same as master cylinder rear outlet).

But to be on the safe side, I'd recommend measuring the old nuts to be sure.

[verona33]

That was very hepful thank you.
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