Loadstar 1850 brakes

Hi, I've read some on this forum, and this is my first post. I was wondering if anyone could offer me any help with the brakes on my 1978 1850 twin screw.
It has hydraulic brakes, I think one of the last years they had them, and I'm not getting enough boost.
To start at the begenning, I bought this truck last fall, for short run hauls of grain off the field, and into town. It has a 466 diesel engine, 5X4 tranny, and vacuum boosted brakes. When I got the truck, the brakes worked well. However, this winter, when it got cold out, the brake pedal was slow to return, and on a couple occasions, didn't come all the way up, and the brakes would drag a bit. By the pedal being slow to return, I don't mean the pedal itself (the spring pulled it up) but rather the action in the master cylinder. I could feel it was sluggish on its return.
With the idea that the return spring may be weak, the bore rough, the leathers swollen, or the cylinder full of sludge, I decided to 'fix it right' and replace the master cylinder. While I was at it, I replaced a couple rubber lines that were showing signs of age.
It seems that ever since then, I have had weak brakes. My first thought was that with the new lines, or during the bleeding process, I may have run the booster into a part of its travel it hasn't been to in years, and roughed up a leather, or tore a diaphragm (it is a double-diaphragm booster, and acts like it only gives half the boost it used to, hence my guess).
Replacing the booster didn't help, so I took it to a shop, where he diagnosed the new booster not working properly. They replaced the booster, bled the lines, and adjusted the shoes.
I still don't have enough boost. Running empty, the brakes act OK, but when loaded, I run out of leg strength, and still can't stop like I should. The pedal goes down a little more than halfway to the floor, and holds, it does not drop, I don't have to pump the brakes to actuate them, just press down on the pedal, and pumping doesn't give me any more boost, it just causes the pedal to be higher up, before I can't push it any farther.
I have good vacuum, almost 30 inches, the check valve holds, and I can see the needle drop, and start to rebound as I apply the brakes.
The mechanic thinks that there may be something wrong with the master cylinder, or perhaps the parts guy gave me the wrong one.

Does anyone else have any ideas on this? I really would like to get the brakes working like they should, as I really like the truck otherwise, and it is in amazingly good condition for its age, and amazingly rust free. When I replaced the brake lines, they came right out, not rusted in, the bleeders worked on all wheels, no rust on the brake lines or anywhere else on the chassis to speak of, and it has under 100,000 total miles, since new (so I can assume the pads are not worn out). I got a screaming deal on it, and when I get the brakes fixed, I see no reason I won't be able to use this truck another 10+ years. However, I also can only justify spending so much.
Thank you in advance, for any advice you can give me, and if any more details are needed, I will answer questions as best I can.

Jerry

someone with experience will speak up. But my first thoughts were the drums or shoe are "glazed" and not giving you the grab you need. Master cylinder, Those with the bigger rigs should be able to help on that. All else, from your description, sounds right.
Good luck, and Welcome to the group.

Did you bleed the brake booster? Have to bleed it first then the wheels. MM

Hard pedal, two things come to mind. 1) the mechanical advantage between the pedal and brakes is wrong. Where it showed up after the master was replaced, could point to too big a master cyl being used. Check the spec's of the one put on vs. the one removed.
2) hard drums. Drums that are old, thin and have been repetely heated can get very hard, this is why there are limits to how much wear allowed.Hard drums are slick, and the brakeing effect is greatly reduced. Check the drums for inside diameter and look for "hot spots" and heat cracks. The only solution is to replace or if not avaiable have a new sleeve installed in the old drum, unless enough "meat" is left to allow machineing thru the hot spots. Someone posted a place that does this work, but be prepard, it costs a lot.

I can't see how it can be the pads or drums, as I had good brakes maybe 200 miles ago, and the weak brakes showed up 'all at once'.
The more I think about it, the more I think it happened when I swapped the master cylinder, or at least very close to that time. I live in a fairly flat area, so running empty, I didn't realize they were weak.
Perhaps the new master is too large a bore, as the mechanic commented how little pedal travel it took, before he couldn't push the pedal down any farther.
Air should not be an issue, as I took it to a certified mechanic, who re-bled everything. The drums never got hot enough to blister the paint on them, although once when they were dragging, they got hot enough to 'stink' a bit, which is how I realized they were dragging. However, I am sure I had good brakes after that event.

Thank you for the replies so far. Hopefully, it will be something simple. I'll keep you updated, how things progress. The shop is busy until late this week, so maybe by end of the week, I'll know more.

Did you use an exact replacement MC & H-vac?

My guess is improper bleeding,with a hydro-vac system it's a must to follow the correct procedure.Was it bled using a power bleeder? If not it may be worth trying it with one.

Here's a link with some good info (trouble shooting/bleeding) that may be helpful.
http://www.bepco.biz/Catalogs/vacuums_&_hydraulics.htm

More good brake info;
https://www.box.com/s/otkihx6yoa/1/61466349


Do you have a Line setting ticket for the truck? It would be helpful to know exactly which system the truck has.

Thanks for all the advice. The guy I first took it to, after I couldn't figure out what was wrong didn't have a power bleeder, so I bit the bullet and took it to the official Navistar truck place, where they have a mechanic that has been a truck specialist since the 70s, and worked on these trucks since new.
He has a power bleeder (the first guy didn't) and found something else wrong (I am thinking a stuck check valve if I remember right). The truck now will easily skid tires on pavement when empty, without pressing all that hard on the pedal. They noticed it was 'damp' when they were setting the pads on one wheel, so I gave them the OK to pull the wheel/drum, and they found a leak on the wheel cylinder. I am going to have both cylinders on that wheel replaced with rebuilt ones, and after they get that done, and the brakes re-bled, I should be good to go for a while. They said the pads were still at least 1/3 there, and the drums looked excellent.
The mechanic thought after this is done, I should be good to go for some time.

Thanks again for the advice.

Jerry

Bad news, the brakes still weren't right, so we pulled another drum, and those pads were worn way down, and worn unevenly. To get the drums trued, would require turning them out to the max, so new pads and drums are called for. Fortunately, they tracked down drums, they'll be here in about a week (the place they are coming from makes a shipment twice a month on a route, so there will be no special shipping fees, as compared to regular freight charges if I want them here sooner).
While it is apart, we are putting in new wheel cylinders and hub seals as well, with the intention of not having to tear into that part again, for the life of the truck.
I asked about just cutting my losses, and trading up for a truck with air brakes, and they said that wherever I traded it, would subtract the cost of a brake job from the trade in value, so I was kind of between a rock and a hard place. I decided to fix it up, because between me, and the last owner, it has had a new clutch, injector pump, injectors, and transmission within the last 5000 miles, and 8 new tires in the last 500. It drives and runs well (466 turbo diesel) isn't an oil burner, and is straight, and very, very rust-free for a truck its age. Its only issues are the brakes, a radiator that leaks where the hose connector solders to the reservoir, and age. For about 1/3 to 1/2 the cost of trading up to a truck just 2-3 years newer, I'll have a solid truck. Also, I have yet to find a truck its age, that is in as good of shape as it is.
Thanks again for all the advice, just thought maybe you guys would like the update.

Thanks for the update Jerry. Sounds like you will have a "new" older truck when all done. If radiator is still an issue, perhaps pull it while the truck is down and have that connection re-Silver soldered. I know that it is always a pull to spend money on an older rig or trade up to a new/newer one. I have always looked at it this way. Once fixed, will the vehicle last me Many worry free years. and I factor in the fact that I own it. NO Payments! Trading in on a Newer Used rig, makes me wonder if I am going to "inherit" some unseen problems that will cost me additional money and I will have Payments besides.
You will enjoy your truck and use it with Pride.

I thought of that (radiator), but the shop manager doesn't want me to touch the truck while it is in his shop (he states liability reasons) and I'll be darned if I'm going to pay shop rates for someone to do a job I can do myself. Any work I can do myself, I intend to. Really, the main reason I'm paying an arm and a leg for labor rates, instead of just getting a dual dolly (and a few other specialty tools), changing the brakes myself, and just paying the guy to come with the power bleeder is for liability reasons. If for some reason, the brakes were to fail, I can say I took it to a certified shop, and had a full brake job done, and hopefully protect myself from liability in a lawsuit. I must not be alone, as the guys in the shop say that they hear people say that more and more, when it comes to people having them do work that effects on the road safety, like brakes, steering, and suspension work. It really isn't a bad leak, I can drive it for several days before the reservoir is empty, but I need to fix it, as it will only get worse over time.