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When you say "fades" do you mean brake pedal slowly travels even all the way to the floor or change in power assist feel?


Pretty basic brakes on this vintage.
Why did you replace what you called a "distribution box?" That should be a multifunction proportioning valve specific to the vehicle but wouldn't cause fade unless it leaked.
The new item mentioned do NOT mean they are good. At a glance suggests bad master cylinder if it loses pedal to floor.
Erratic power assist is a vacuum issue w vacuum stored in booster and locked in booster indefinitely even with engine off should be holding vacuum inside and proven by at least one power assisted brake pedal push with engine off for a while. Faults include bad booster, check valve right where hose goes into it and the grommet.


Just FYI on the proportioning valve: It's job is to direct pressure/braking force for the vehicle it was installed on or meant for. Trucks are generally intended to apply more force earlier to rear brakes and you have a mix of truck and car style. At the vintage many auto makers would or wouldn't offer OE power assist at all or option of front disc brakes for certain powertrain options.
I'm never going to find what would be OE for the chassis on this what was originally offered. Doesn't all matter as brakes fading if sitting still isn't correct but could be confused with changing vacuum assist.


You would know that by shutting engine off, apply brakes till all vacuum is used up and brake pedal feels harder. Then hold brake and start engine you should feel pedal drop some with now vacuum assisting when it wasn't before. If not or erratic with stable engine speed there's probably a vacuum source and sealing problem with anything about it - engine itself on thru all items including the booster,


T

It's not slowly, it's pretty much to the floor right away. It's almost like I'm not getting any break assist. The reason I replaced the distribution block was because after I replaced the master cylinder/booster, I pressure tested the system and wasn't getting anything out of the port that feeds the rear. It is not a proportioning valve. The OPGI part number is CH26980 if you want to see what it is. The intake manifold is pulling vacuum. This is the second master cylinder/booster that I have replaced. Unless I got 2 in a row that were bad.


C

? OK, whatever distribution box replaced is fine. It has a proportioning valve then also that slides inside to direct fluid pressure AND if off center meant to lock off leaking or unequal pressures plus on a 1968 get a red brake warning light. When those are far enough off center they won't bleed out fluid unless centered again by releasing pressure from the one end that works the shaft inside moves to allow flow to both front and rear in this case. A good one self centers by leaving it alone so you don't hear much about them.


Now you say you feel like you have no power assist possibly due to lack of vacuum or ability to hold it. Vacuum source is intake manifold actual vacuum on this via a usually a rubber hose that will not collapse and stay on ports snugly from below carb itself on to a plastic check valve at the booster. It can fail anywhere it leaks or if not locked in check meaning vacuum only travels one way thru a check valve. One would think booster would come with a new grommet but might not be correct? It just that silly rubber thing leaked your brakes would lose power assist - that simple.


Some guess work: I would bet master cylinder has an adjustable rod to booster. If wrong that would leak out vacuum without touching anything as if you were pushing the brake pedal just a bit. Specs for that or jus know that by feel of parts.
Multiple bad parts. IDK - by the second bad part you have to seriously question if you wrecked it or some other reason is wrong. No question bad parts exist new or plain wrong parts.
This is 99% a Chevelle as HT said NOT based on a truck chassis. Changes known OE of a chassis include which engine, body style as to whether it gets disc front brakes (not sure for 1968 on this) or drums on front, proportioning valve, size of brakes. If those matched and were all wrong you would still have brakes just not what it was designed for.


Side note if you find this the or part of the problem. Shaft adjustment to the master cylinder from the brake pedal I think is under the dash above brake pedal. A thing you never needed to touch unless altered or damaged.


Also: If this isn't bench bled properly you can end up with air you'll never get out without starting over or pressure bleed system.
Air in system make a variety of odd behavior also even small amounts very temperature sensitive as the air will expand with a running engine and can lose all brake feel that might come back some when cold,


T

Thanks for your response. I went and bought a hand pump and bled it out again. When I was trying to figure this out, I blew out the lines. The hand pump worked.

Whatever tool you bought to bleed out brakes if it worked and all is well now can't argue with that. Open for now but thread should probably get locked up for archives soon as a solved issue,


T

I'm going with drum brakes around till stated otherwise. OP - Master cylinder would be different if for a disc brake vehicle. Larger reservoir towards booster the clue, if same size it's correct.


Where parts new or rebuilt - master cyl + or wheel cylinders? Matters. Rebuilt had/has high failure rate. Wheel cyl (rare for this) could draw air back and not leak with pressure.


Notes for drum brake musts: Shoes correct and star adjusted at bottom to slightest drag or drag then back off to none. Do not adjust parking brake (rear only) until shoes and all parts are exactly correct and adjusted. If wrong service brake will never work right either spongy or burn up by themselves. Some could blow a piston out of wheel cylinder - most can't but never feel right.


Adjustment in order of events must be first before bleeding.


Note: Drum brakes look intimidating with springs and things + double as parking brake well if right. If confused Google out pics as they are still made now for other use (trailers etc.) No fundamental changes in principle operation for ~60 years!


Any hydraulic items using rubber if dry will tear up rubber and fail almost right away might be part of repeat problems?


Know parts (most) at wheels are for "towards front or rear" or for "left side right side" can be installed opposite and fail but not total loss of pressure feel sitting still,
T

Sounds to me like you just don't know how to bleed it properly. You probably need to have it pressure bled.

Ditto HT - Twas long ago but I recall that many GMs the master cylinder was at almost a 45 degree angle as bolted to booster was a b*tch sometimes.


Tricks of the time: Floor jack hoist driver's side of car much higher than passenger's first. Vacuum pull out some fluid out of bleeders then thru a clear line so you could see it then proceed.
I'm almost certain NOT for GM anything but Fords you had to push a red rubber button on proportioning valve or it was stuck on either front or rear only! That centered it if spring inside (they all were fine then) could move. If rusted from old fluid maybe fuss with it or find another - very difficult to get right proportion of pressure delivered.


Other: I don't think GM put disc brakes on anything but Corvettes in 1968 except for experimental by special order on some Oldsmobiles - owned one - 4 piston Vetter brakes really nothing special. Cadillac didn't offer disc brakes in 1968 TMK on anything!


T

This vehicle isn't a truck. It is a Chevelle chassis with the back cut off.

If you have 4 wheel drum brakes and the pedal drops after initially having a good pedal and it has no external leaks, the problem can only be the master cylinder.
Just make sure you are confusing the symptom with just a spongy pedal due to air in the system.

Is it only happening on one side?
Could you have swapped sides with the adjusters?

no, I havent done anything but slip the adjuster to loosen it,put wheel back on, and in a few days its tightened up again.

thanks for ur reply

only 70,000 miles, but thats an idea. Thanks alot. I'll check that next.