O.T. '64 F600 - Hydrovac Brakes

Well the ole girl has been sitting awhile, and I find no pedal on the brakes,(or kinda to the floor !@#$&%! LOL !) Thats better than finding out on the road. I need to move it back here, 30 miles or so.

Question is, or I suspect, diaphragm in the frame mounted hydrovac unit, or what ? I bled the brakes starting at the furthest cylinder and never got any real pressure, sputtering before a solid stream out of that one, refilled the master cylinder, did get a small stream out of the next over, on the rear axle, but no pedal, I'm thinking its the hydrovac assist. There is an air filter or I believe thats what it is in the cab up by the top of the seat with a hose, I do not hear any air moving, you could before, hence my suspicion of the diaphragm, I've never worked on one of these on the frame, but plenty of other similar brake systems with the booster on the firewall behind the master cylinder. Master cylinder was low, so I'll bet a wheel cylinder is leaking, but I saw no sign, so its slight, I used to have to bleed them periodically.

I'm not sure how to ID it on the truck, but it appears to be similar or just like this one in the photo below and looks old, though been fine, age, sitting etc. It appears that I may have to match up some "plumbing" maybe there is some marking on it, its rusty, I did look up the one below and found it listed by Haldex, but again, looks the same, have not confirmed, hopefully these are not outrageous in price LOL !

I'm assuming from what I have looked up on the web, you would replace it, could rebuild if one wanted to thought I saw kits, but given its a brake component, never been inside one, the previous may be wiser than the latter LOL!

Ideally, I really want a double reservoir master cylinder instead of the single, so at least you have one axle with brakes if a hose or line was to rupture, anyone know how to do that on these old trucks.

I don't use it much, nor do I ever put any weight on it, its just handy to have though, might be time to go through the brake system and know they're done.

PS I do have the factory service manual, will have to read up.


Haldex

I rebuilt one off of an early 70's Ford cabover some years back. The vacuum cylinder was made a bit different, more like a regular cylinder, but it still worked pretty much the same way. That said one thing I ran into problems with on it, that caused it to not work properly, was the check valve between the engine, where the vacuum originated, and the cylinder. Can't remember exactly what it was, or wasn't doing, but I do remember having to change the valve to get it to work right. Might check on that if your setup has one.

Wayne that sounds like valuable information, I assume it works off vacuum from the engine, so that in between check valve could be an issue, or one of the obscure things overlooked, and we've all been stumped at some time having done mechanical work, will make sure to keep that in mind, thanks for posting that !!!

Yes, open the vacuum line at the hydrovac with the engine running to make sure that there is suction to the hydrovac. The engine will run a little rough with an open vacuum line, but will run smooth when you reconnect it, providing there isn't a leak in the system somewhere.

Will definitely try that, thank you !

Even with a bad Hydrovac you still should be able to get a hard pedal, you just won't have any boost. If your pedal goes to the floor you have air somewear in the system. With the small volume of fluid in the master cylinder it doesn't take much air for the pedal too goto the floor.

Will keep that in mind, air bound, its possible the last time the moved it out of the arena building, as my father I think drove it said it had no pedal, master cylinder was down a lot, and if that pedal was moved, there's the air, but I did get a solid stream, from the drivers side wheel cylinder, just a trickle from the other side of the rear axle, never got to the front ones, and I think those are seized, in the past a little bleed off, pedal was always all the way up brakes seemed to work well.

years ago i had a hydrovac go bad--the diaphragm burst and the tank had a lot of brake fluid in it. Had removed the check valve a year before and the engine sucked up and burnt all the brake fluid--tons of smoke. Later learned from a truck brake shop that was why the check valve was in the vaccum line.
This was on a 1955 ford NYS Drill Team class B racing truck that i had built.

I have a 74 Ford F600 and A 63 Chev C60. The C60 set for a few years and had no breaks. I started to town with in and my brother was working the hand brake. About four miles the brakes started working. After that I would let in run for several minutes and then pump the brakes and they would come back. Now I drive it about once a month and they work find. Someone said it was the rubber in the master cyl and just dried out.

The simplest way to bleed a hydrovac unit is to put a pressure bleader on the master cylinder. (this supplies brake fluid and about 50psi to the cylinder. Now every wheel cylinder, and bleeder on the Hydrovac, will allow fluid to come out with no pumping. The frozen bleeders can be blead (from the hose in some cases, or taken apart at the drum to allow air to be removed with the piston removed from one end. (do not use the power bleeded for that!)
a hydrovac cannot be judged as bad untill it has been bled of all air. It just will not work, and appears to be bad. Do not condem it till bled. Jim

Actually the check valve in the vacuum line is to prevent gas vapors from the engine from getting into the hydrovac and ruining the rubber diaphragm when the engine is not running. Rebuilt Hydrovacs come with a new check valve.

The most important function of the check valve is to maintain a vacuum in the booster while the engine is under load. While an engine may run with about 21 inches of vacuum while under no load, under full load there can be no vacuum available. Without the check valve if you needed to make a quick stop while the engine is under load you would have no assist from the booster.

To check the operation of the check valve, run the engine and then shut it off. After the engine has stopped you should have at least one full brake application with assist. Without being used a good system will hold vacuum at least several days.