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My rear brakes lock up before my front brakes and the mechanic says it needs a new proportioning valve. I can't find it in my haynes manual so I don't know what it looks like. Is it something that needs to be replaced or just adjusted? Any help is greatly appreciated.

On a 2 wheel drive, it's mounted on the drivers side fenderwell on most trucks. It's the thing with the brake lines running to it. I believe the stockers are nonadjustable. It's pretty uncommon for them to go bad, IMO. Is the truck all stock? If not what mods have been made. If you trust your mechanic, I just removed the one from my truck as I reworked the braking system for 4 wheel discs. You can have mine for the cost of shipping...

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Proportioning valves don't normally go "bad" what happens is the piston inside move to one side from either a fluid leak or a bad master cyl.
If the prop. valve is having a problem then the brake light on the dash will come on.It is hooked up isn't it?
This said there is a fix.
First make sure your master cyl. is in good operating condition and if so then the tricky part.Pull the light switch out of the prop. valve then get a good light and look in to hole there is is a valve with a cut out in the middle this cut out is where the nub on the light switch is supposed to be if it is to one side you can get it back to center with help from a friend.If you can see the groove on one side this make it easier but if not it just takes longer.Crack open the line on the opposite side you see the groove and have somebody push SLOWLY on the brake pedal while watching for the piston to move when it gets to center have them stop close the line install the switch and your back in business. If you can't see the groove then you must try it both directions until you get it center.

Rule #1 make sure your master cly. is good
Rule #2 Make sure the brake light is working (Ground the wire on the prop valve)
#3you can bleed the brakes starting with the front in this case and it might do the job for you.
hope this will help you and i will stay tuned to see if you got it working.

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1978 pro street step/side 455Buick, 4sp,4.11s, 31/18.5s

I checked out the proportioning valve and it looked okay and the mechanic didn't think there was anything wrong with the master cylinder. He said it was unlikely the metering thing was out of whack. He loosened the adjustment on the rear and it feels fine I just have to do a few more hard stops. He recommended bleeding the brake lines but he said there was a specific order to doing it if you run some kind of cleaner through it to clean out the gunk. I've never done this before but I think I can do it. Has anyone else done this where you put "cleaner" through it? Or would it just be fine to flush them out with new brake fluid? In that case do you just empty out the old than fill it in with the new? If you stomp on the brakes hard enough shouldn't all the wheels lock up because only my rear ones do but not my front. Last question, is there a certain size hose that fits into the drain thing well? You guys are great, thanks in advance