You can get adapters or different thread size line fittings. If the combo valve was made for 1/4" you should probably use an adapter. NAPA sells them.
I doubt you would have a problem with the smaller line. Let me know if you do though.
I have seen a lot of vehicles with 3/16" rear line. The above info I should have specified was for 67+ GM RWD drum/drum and disc/drum dual circuit, with which I am the most familiar.
I don't know why some manufacturers went to 1/4" for the front to rear line. Sounds like it is for volume, although I'm not sure that makes lots of sense, hydraulically, considering the rest of the system. I have been trying to figure that one out myself. I also have no idea why some had the front circuit of the master for the front, and some for the rear.
One note on the combo valves... the sections are designed with specific vehicles in mind. The truck one is likely a good choice for your Buick, as the weight transfer vs. cylinder sizing may be very similar. There are ways to tweak the balance if you need to. If the rears dont seem to do much work, the first thing I'd do is switch to the larger line though.....
Last edited by yesgo; 12-10-2004 at 04:37 PM.