76 d700

[70chall440]

New to this forum but not to Dodges; I am a little new to large trucks however. I have a 76 D700 that I rescued and as a part of that had to replace the master cylinder; I could not find an "exact" match, however I did find a Centric MC that was supposed to work. I got it in ok, but I am having the problems with the front brakes locking up. They don't stay that way, they do release slowly. I thought that the push rod was too short and applying pressure on the MC, but when I looked, it seems to have some room.

The MC is the same as was used on motor homes of the same era and comes up as a IH unit. While I am not ruling it out, I am having difficulty figuring out how this is happening. I am going to bleed the brakes and see if that helps. The one thing I don't know about is the air system and how that is affecting the overall situation.

Any help, suggestion, etc is deeply appreciated.

 

[Steve Frazier]

Check that your rear brakes are adjusted properly too, that could have an input to the problem. You didn't mention the condition of the rest of the braking system, if the rear drums are worn beyond spec that could give you the problem you described.

 

[70chall440]

I haven't had the wheels/tires off so I haven't looked at the brakes directly at all. The old MC was leaking which is why I replaced it, now I am having this issue. that said, with the old MC the brakes released properly so while I would agree that the brakes may be worn, I should not be experiencing this issue as a result of that wear since the only thing I did was change the MC. My belief is that my current situation has to do with the MC directly; either it is incompatible with my truck or it's exchange triggered some other problem. I am very familiar with passenger car braking systems, however the air component of this truck is new to me, so I am wondering if the air part is part and parcel to the overall condition; meaning perhaps the new MC is not compatible with the air system in some way.

 

[Steve Frazier]

So you're saying this wasn't happening before the repair?

 

[70chall440]

that is correct, brakes worked normally. I changed the MC because the rear seal was leaking brake fluid into the cab.

 

[RonG]

There is a rubber one way valve called a residual valve in some master cylinders that can hang and cause problems,not to say that it is your problem but I would want to know that it is working properly.It is a 5 cent item but can be a PIA if defective.I thought that you were going to tell us that you had a BigHorn or something.Ron G

 

[70chall440]

well.. actually I do have a 2014 BigHorn but that is not what I am working on at the moment.

I am assuming that this one way value is inside the MC and would require it to be disassembled? I am have a hard time getting my head around this, I is a fairly simple system and even if the MC is for something else (like a IH school bus) it would work the same as all of them. I could understand if perhaps one set of brakes was engaging or locking sooner or later than expected but not what I am seeing. It is like the pressure in the front brakes is holding and releasing very slowly (actually that is exactly what is happening). I cannot see how changing the MC would result in that. I suspected the push rod length was part of the problem and adjusted it slightly, but after looking at it I don't think this is the issue as it "might" have been one turn out at worst. I am now thinking it is more of a contaminated fluid issue or perhaps a prop valve or something. Again, since I don't understand the air aspect I cannot factor that in to the diagnosis which is what led me here.

 

[RonG]

I think that the valve was just inside the line out of the master cylinder and back when I encountered it there was probably no dual braking systems like we have today.There used to be a law that you could not rebuild master cylinders and you could not buy the kit to do so but I think that has changed now.I was talking about the BigHorn of the old days,the over the road tractors,too bad they don't make them anymore.Ron G

 

[70chall440]

I believe for modern MCs, it is an issue of the piston and port alignment; however I am not 100% certain. I have it in my head that perhaps this MC applies too much pressure to the front brakes which is resulting in the brakes sticking and not releasing. I am contemplating installing a proportioning valve.

 

[70chall440]

I have decided to put a proportioning valve into the front line and see if I can dial it back.

 

[70chall440]

The proportioning valve did not work; after a lot of investigation I have deduced that the booster is suspect. Every issue I have comes up pointing to the booster. My research shows that the booster is a bendix single diaphragm PN 2512065 which crosses over to a cardone 51-8000. Problem is that every where I look it shows this booster only used on 60's era GM trucks so I am a little reluctant to drop $200 on one (they are plentiful). I cannot find a kit to rebuild the one I have (yet). I really need to find a brake guy; talked to one guy yesterday but he was very focused on selling me a new MC.

 

[RonG]

It seems like the only thing that would fail would be the diaphram in the booster as long as the stem is adjusted properly.This is getting interesting now.Good luck.Ron G

 

[70chall440]

Actually there is quite a lot going on in the booster; in my case it appears that the return spring is not doing it's job so that when the vehicle is running the vacuum is allowing the diaphragm to apply pressure to the rod which is in turn applying pressure to the front brakes and locking them up. This "may" be result of several issues to include but not limited to broken return spring, corroded rod, blocked check valve, etc. Apparently you are supposed to put 1oz of non-petroleum oil in the booster annually. I need to remove the vacuum line and confirm that the brakes work normally (albeit manually). if this checks out, then I will move on to bleed the system and see what that yields; if it still doesn't work, then I need to replace or rebuild the booster.

 

[RonG]

Keep us updated.I am so old that I will probably not ever use what I learn but it is hard to break old habits.Ron G

 

[70chall440]

I will let you know what I discover. Working on a truck this size is like working on a rare foreign car, everything is hard to figure out.

 

[70chall440]

Now I am at a bit of a loss; I disconnected the boosters (disconnected the vacuum line) and tried the truck; got the same issue with the brakes locking up. This leads me to the conclusion that perhaps the one guy I talked to is correct, that the MC is responsible for this condition although I find it hard to understand why this would be. I still need to bleed the system and perhaps this will yield some results as I am sure the fluid is contaminated with water and most likely has some air in it.

 

[RonG]

I agree on the MC being the culprit.That residual valve that I talked about could cause that.It is a little one way rubber doohicky just inside of the brake line that controls the return of the brake fluid in coordination with the valving in the MC or something close to that,of course it has been a long time since I worked on one.Ron G

 

[70chall440]

I agree is must be a valve as the bore is the same. I pulled the old MC apart; not much to really look at, however you can see/tell that the rear seal was slightly deformed I suspect from rust and corrosion over years. I am going to bleed the brakes and see what that does.

 

[70chall440]

well as with most things in life, there is good news and bad news; out of the 10 bleeder valves; 7 of them open, however 3 of the four in the front will not budge and of course immediately rounded off despite my slow approach. So now I am not sure what to do, I am hesitant to put any heat/flame on them as I am quite sure I would have a raging fire rapidly. Cannot really get a vise grip on them due to their location and the fact I am pretty sure they will just round off even more. I had soaked them penetrating oil some time ago which didn't seem to help too much. I really do not want to pull the wheels off (huge) and I have a feeling the wheel cylinders are like the MC, made of ground unicorn horn (i.e. expensive and hard to get). At this very moment I want to drag it into the field and set it on fire or sell it for parts...

 

[70chall440]

Todays update - I somewhat bled the brakes (7 of the 10 bleeder valves); definitely got some contamination out of the system; unfortunately the front bleeder valves were/are stuck and subsequently I could not truly bleed them. What I did do is crack the lines and tried to bleed as much as I could that way; the result was no change. After staring at it awhile I decided that I needed to pull the MC and take it apart to see what was different from the old one to the new one that supposedly is incorrect. I got the new one out and disassembled it. It is identical to the old one with one exception; there is a thrust washer on the end (where the push rod goes in); this measured out at .175" whereas the one in the new MC measured out at .185". Regardless, I disassembled the new MC and removed the seals and put them onto the internals from the old MC, then reinstalled everything into the new MC housing. After bleeding the MC I tried it again; still no joy however I concentrated on what the truck is doing and not doing; here is the synopsis;

1. with the parking brake on, if you put it into gear the truck seems free to move forward or backward.
2. once you touch the brakes; it "appears" the front is locking up
3. under idle while in gear (either direction) with your foot off the brake, you can feel the truck inching in whatever direction you have the trans in
4. if you are backing up with the truck essentially dragging on the brakes and put it into drive with your foot off the brake, the truck with jump forward, if you touch the brakes then they grab immediately and lock up
5. it "seems" that if you pump the brakes, they want to release a little

what I am thinking now is that something in the front brakes in dragging, not releasing, etc. This of course will mean I have to take the front tires off and get the drums off; not an easy thing to do. I do not think it is the MC as the guy in MA told me as I just cannot see how it could be. The only other thing I think it might be is the distribution valve located on the frame but seeing that everything is getting fluid I am not convinced. I am "thinking" that if I take the drums off, I should be able to see the brake shoes pushing out and retracting (the retracting part is what isn't working).

I have given a lot of thought as to why this has happened when they seemed to have worked before I replaced the MC; well my theory is that just prior to moving the truck into the shop, I discovered that the MC was empty due to it leaking into the cab. I think that there wasn't any fluid or barely any in the system and the brakes were barely working. When I replaced the MC and closed the system, it pushed things further than they were used to going and subsequently resulted in the brakes dragging.