The Stovebolt Site Forums The Parking Lot Big Bolts is this the way my brake pedal should feel with hydrovac??

I just replaced the master cylinder and blead the brakes on my 1970 C50. The master was shot leaking out the back. I have never driven this truck before, as I just purchased it and trying to get it road worthy. The brakes work but when you first step on the pedal is is rock hard for a split second, then seems to have some boost but the pedal dosen't go very far before the brakes grab. The truck has a duel hydro vac system and all the vacum end appears to be in good shape. I am not use to this type of truck/ brake system and wondering if it sounds like it is working as it should?? How can I tell if both hydrovacs are working?? I know it will feel different compared to a regular pickup.

I don't know of any trucks around here that I can compare mine to. Unfortunatly most have seen the crusher around here.

I am a novice at these things, but my truck has a single hydrovac. I redid the brakes, my pedal feels kind of soft. I usually pump it once or twice, mine seems to work quite well. Their the best brakes I have ever had on an antique. When I have really had to stop, it has stopped short every time. The system has impressed me.
I am not sure, but it sounds like yours is working. I would try driving it around abit, just be careful, pick you road careful and you well find out quick how their doing, maybe get a lot of confidence in them, good luck. I am learning so I will be watching the replies as to what others say, Tom.

me too!
Tom

rust master,

I have driven many trucks with hydrovacs...they shouldn't work as you have described. If they sit for any length of time, the brakes are touchy when first driven. I have always attributed this to dampness or light flash rust on the drums. The pedal feel should be almost like a conventional vehicle...no hard spots or binding.

I used to have a Midland-Ross troubleshooting card, but i can't find it. I did a search using 'Bendix Hydrovac' and found their troubleshooting guidelines, but didn't see your problem listed.

Something isn't working properly...I don't know what. Good luck and keep us posted!

Stuart

Never driven one of your trucks but...I added an old hydrovac from a GMC big bolt to my little 3/4 ton. The brakes were redone including new front drums. The pedal was hard before and I had to stand on it to get it to stop. With the hydrovac, the pedal is soft, has a fair amount of travel before the brakes engage. The softness and travel throw me off but it stops like a mofo.
http://picasaweb.google.com/orchidskip/HYDROVAC?authkey=57r0_h37YH0

Thanks for the info, The pedal is definitely not low, seems like there may be a problem. it does not have a ton of assist as you have mentioned ( I did find 2 sm vacum leaks today when I took the carb off). I'm wondering if one of the boosters are seized, or a seized wheel cylinder. I haven't driven it on the road yet. I may try a short trip down the street this weekend to see how it stops on the road. I am just waiting for a exhaust doughnut to put the front pipe back on.I should have that tomorow. I'll keep you guys posted as to how it stops on the road. I guess my biggest ? is how can I figure out if it is the front or the rear brakes/booster that is the problem??

Are the hydrovacs on your truck set up one for the front one for the rear? Also maybe jack the truck up first, back axle and put it in gear and see how the bracks do, then try the front. Might be better than a road test. Just make sure you got it blocked up good so it aint going no where. Again I am a novice but thats what I would try.

rust master,

I'm not an expert on these brakes, but are you sure you have separate systems. Doesn't your truck have the hydrovac unit mounted under the cab on the frame rail. It might have a single diaphragm, or a dual diaphragm, but I think it's only a single source of assisted braking for both front and rear brakes.

Stuart

Yes, but it has two hydrovacs units. one on the out side of the frame rail and another hydrovac on the oposite frame rail. It has one vacum line from the intake to a check valve on the firewall then a hard line down under the cab, where it tee's off to each hydrovac unit. both are under the drivers side . must have been a option for 2, because most people here have told me their trucks only have one for the rear.The setup looks factory.The truck is rated total GVW at 26,000 lbs. Both units are identical and look like bendix units.

My truck is a dump truck now, but the dump body is newer and I am starting to wonder if this truck was something other than a dump earlier in life. ( I cannot find any signs that it was though) Mabey a fuel/oil truck, that might explain the 2 hydrovacs. but it only has a 292 and NO 2 speed rear. I don't know and the previous owner bought it the way it is now. ?????

Also, My truck has a DUEL resivoir master cylinder 2 lines out. That may be causing the confusion with some of you older truck guys, I know yours are single resivoir masters. Sorry for all the posts, I just keep thinking of things.

Dual resivior and dual boosters?... Wow, whats the GVW on that ol thing? Thats a lot of oomph!

I work with similar units on aircraft, mostly made by Bendix oddly enough, just for different systems is all. Seems to give me a little more insight into the operational design of them anyway,...

If I had to guess, & I do, I'd ask if you made sure you bled the air out of the tops of the HydroVacs where the pneumatic relay is?
Looking at how the system works, if there were a good sized bubble up there it would reduce the reaction time of the relay & thus when & how rapidly the booster engaged, hence the soft pedal into hard braking. Rust in the relay piston bore making it hang up could do the same thing.
Might also be a simple case of needing to adjust the slack out of all of the brakes & trying again. The relay piston relies on back pressure to move it, if there's too much travel in the brakes the fluid takes the path of least resistance until it builds up, so it would take more push before it moves.

Again, I'm not terribly expreinced with these old hummers, just offering my ed-u-mikat'd observations from other similar units & rebuilding my HydroVac system from the ground up.

Freq2002, Are you saying that aircraft use a similar or same deal as hydrovac? If so does this suggest that they are a pretty good device?

http://picasaweb.google.com/orchidskip/HYDROVAC?authkey=57r0_h37YH0

I'm looking for more reasons to love this upgrade other than the fact that is works really good!!!

Aircraft use a similar actuator on severl types of valves for all kinds of systems, and Bendix is by far one of the largest manufacturers of those items.
As for the device itself, I think that the fact the basic design of these things has not changed from way back when until today since the same units are still in use in heavy forklifts.

There are many similarities between the Hydrovac & many large pneumatic actuated valves, only difference is instead of pushing a valve shaft, the HydroVac pushes a piston. Otherwise the operation is much the same.

My truck is rated at 26,000 gvw. seems prety std gvw for a c 50. I dont know why it has 2 boosters. I agree it seems like over kill. I have a idea thjat my problem my be in 2 of my brake hoses. There are 2 brake hoses just after the master cylinder, where the lines go down to the frame. They are used to compensate for body/frame flex( where most trucks/cars would have a coil of line under the master) When i was bleeding the brakes even with a bleeder screw open I had to apply a lot of pressure on the pedal to get fluid out. I did get clean fluid out, but had to apply a lot of pressure with mu foot. I'm starting to think that one or both of these hoses cave colasped internally causing a blockage. This would cause my hard pedal. the pedal is like standing on a rock at first, then slowly go down and you can hear the booster/s trying to work.I don't think there is any air in the lines because I ran a lot of fluid through the lines and it was good and clean. The only other thing I can think of is that the boosters are seized? we have a bad storm here now so I can't work on it till next week when the weather is better. I am going to replace the hoses I talked about and go from there. They look to be original and are due to be changed any way.

Sounds like a good idea to go thru and replace flex lines, rebuild master and wheel cylinders and repair any steel lines that look bad, new shoes and drums. The hydrovac units are pretty durable so eliminate everything else first. I think the system is designed so that if the hydrovac unit doesn't operate, it is bypassed and the brakes will still operate off the master alone, although you may need to stand it. I think the hydrovac also has an intake air breather filter. They are usually mounted on the firewall on the pasenger side. It's a small oil bath filter. I'm guessing it allows the hyrovac unit to gulp clean air. Last resort, rebuild the hydrovacs. Brake specialty shop near me rebuilds them for about $300 clams so it ain't cheap. But what's a little thing like brakes?

I think all the other lines have been replaced previously, the only ones that look suspect are the ones under the master. The master is new.

Ok, I got the two hoses off today and just as I thought they are plugged/colasped internally.I love it when I find a problem,it would have sucked if I got them off and didn't find a problem because I had to ruin the lines to get it apart ( rusted). Lines are no big deal I've done lines on just about every car I have owned. When you own old cars in the north east you have to get friendly with a flare tool. Now I have to get my parts guy to match them up. I have had trouble finding parts for this truck around here, because there is not much of a call for them. people look at you strange if you ask for parts for anything older than 10 years around here, and ALL my cars/trucks are over that

I've got a 66 c60 and I had the same problems with the flex hose between body and frame. They were totally plugged with gunk...? I'm sure all the lines are suspect. I had trouble finding a replacement at the parts store, but got one made up at the local hydraulic shop. flex
The other side was plugged too but I was able to get the line off, I bored it out with a welding rod and solvent. I'll get another flex line soon though.
Good luck!
Bill

WCKanner your truck is very simular in set up as mine 292 etc just the next body style. My parts guy thinks he has a listing on the right ones, matching up to the book. He ordered them and I am waiting for them to come in (hopefully this week)

Got the new hoses/lines on today. The brakes are much better but still hard at first? once the pedal starts to go down they are fine. I just don't understand why the pedal is rock hard at first stepping on the pedal???