Technical Need some help with brake issue

That is not always true. I repaired an early '70s AMC product that was brought to me for a major braking issue; one front brake wasn't getting pressure. The problem proved to be the pressure differential switch in the combination valve. The car had a hose failure, after replacing the hose and attempting to bleed the system with no success, it pulled violently to one side. After attempting to 'reset' the valve, I found this particular unit wasn't resettable (as per the factory manual) and cut off pressure to one wheel, it had to be replaced. No replacement or subs***ute was available, it was discontinued.

I removed the valve in its entirety, problem solved...

If you need proportioning on the rear brakes, install a separate, manually adjustable unit. Those aftermarket 'universal' combination valves with a pre-set 'generic' setting are useless.

 

Simple test. Next time it happens (make it happen) instead of cracking bleeder do this. Loosen both nuts a couple of turns out that hold the master to the booster and see what happens then report back.

 

Thank you all for the ideas, I will be the 1st to admit that I'm not a mechanic, but I'm trying. I'm at the car and have found the push rod and it is adjustable. I'll attach pics. If I turn it out, towards the front of the car, making it longer, should that allow the return of fluid?

Am I on the right track?

 

I would look at this adjustment between the booster and master cylinder https://www.bing.com/videos/search?...2C4829803C4E91CE88D72C4829803C4E91C&FORM=VIRE

 

Grab, with your hand, the pedal in the up position. If you can pull it up a little before something stops it ( eg underside of floor) the return spring is either absent or insufficient and the weight of the pedal is riding on the master, and this can result in your problem. Whilst down there, now ever so gently depress (more like shake slightly really) the pedal again with hand, which will be against the spring (***umed to be present!). You should be able to detect a small (eg 1/4") movement (of the pedal pad) before the clearance is taken up and the master piston movement starts. No such clearance and the master is not returning fully, again explaining your problem. Your foot won't feel this but your hand should. Use knee pads.
Chris

 

I would think you would want to shorten the push rod. not lengthen it.

 

Could it have been it was only inhibiting full flow? That it was not able to be centered because of filthy fluid/corrosion or failure? And wouldn't both front circuits be affected rather that just one? Seems like a costly component in that after activation it is a throw-away.

I have heard of a GM VALVE that does this but never came across one or read of a direct application. Do you remember what brake system the AMC was using as they used different systems (FUEL - IGN for example) from different car makers.

Really, (IMO) all this system needs is a DISTRIBUTION BLOCK, a front METERING VALVE, an ADJ REAR BIAS VALVE (as much as I hate them but what are you going to do?) and ensure there is a RESIDUAL PRESSURE VALVE on the rear circuit. I see no need for a PDV. The safety factor design is in the MC.

If this (COMB VALVE) was a part of a kit, I would like to know how they arrived at knee point and slope for just this application.

 

You will need to shorten the push-rod if adjustable. The AMAZON video explains the theory correctly. That gap measurement is crucial, too long and you have what you have, too short and you will have a low pedal and slow braking response. That air-gap setting should be in their INSTRUCTION SHEET. There is no close here.

Again, did you buy the combination MC - BOOSTER as a kit or did you use individual service parts? If a kit, there should be instructions on adjusting the push-rod and the required air-gap. Did the MC - BOOSTER come pre-***embled? If it did, the booster to MC push-rod adj. should have been set by the vendor.

When you make the correct MC push-rod adj. and the brakes keep dragging, you will have to check the BOOSTER to MC rod adj. All of this in lieu (Fr.) of gauge(s).

Watch the video.

 

Well I think I fixed it, with everyone's help. I used a couple shims/washers to space the master cylinder back a bit.

Took it for a quick dive (same route as last time) and when we got back home, the front tires did not lock up!

Going to go on a longer drive Monday, but for now I'm feeling a lot more positive than the last 2 weeks.

Thanks again all

 

The MC and booster came as a kit, but that was about 7 years ago... I'm going to keep driving it as is and see how it goes. Pedal feels good and the damn thing, for a 100yr old car, stops pretty quick and straight!

 

- !!! ENJOY BROTHER !!! -​

 

Never mind Too slow. Glad it is fixed
Happy Motoring.

 

Believe me, all of those things were checked multiple times; this car almost drove me crazy. The non-resettable info was gotten out of the factory service manual once I got my hands on one. Keep in mind this was over 30 years ago, but it stuck in my mind for the above reasons. This was an early '70s AMC Gremlin.

As to what brand or type valve it was, I can't say. I do recall though that the valve was smaller than the typical ones you see, with pretty much the same layout but only about 2/3 as long. And it was a sealed unit, no dis***embling for cleaning or repairs.

After owning multiple Jags with their 4W Discs that worked great with none of this plumbed in, just direct lines from the master to the calipers, that made me realize that those 'generic' combo valves couldn't possibly be doing much of anything important. Same with the Gremlin; the brakes worked fine after removing the valve, just no more warning light. When asking about the valves from the aftermarket vendors, all I got was they're 'needed' but couldn't get a clear answer as to why...

I have installed adjustable proportioning valves for rear brakes a few times, but strictly on a case-by-case basis.

 

No, you were correct. The OP only masked the problem but he did realize what the problem was.

HOPEFULLY, he will make the correct repair before something not nice happens.

 

IT will do what it should now.
More than one way to skin a cat.

 

That would tell you that those who say that your pushrod is adusted out too far or that the pushrod between the booster are correct. The shims should be just a temporary fix until you figure out what the real issue is and correct it.

What you had going was actually the same as pumping up and holding the brakes except that the linkage was doing it rather than it being done intentionally.

 

The rod in the booster that acts upon the master is normally adjustable. Once you have got your head around measuring with depth gauge (calipers) and straight edges it is simple enough to measure and adjust, or you can spring a small handful of dollars and purchase the measuring tool. You can do it by feel with a big of estimation thrown in.

Chris