Hey Gang,
I've got a 1973 (1972 chassis) Barth 25L on a P30, with a 402 Chevy big block.
The trouble I have is intermittent. When the vacuum assist is working, it has great brakes, but when it doesn't, they're rock hard. Underneath the body, tucked into the frame rails are 2 chambers that look like vacuum assist chambers you'd find on a big truck, except they don't have any linkage to anything. Are these extra vacuum reservoirs? And would you think my problem stems from the vacuum booster mounted on the firewall (with the master cylinder), or from a sticking valve in one or both of these "auxiliary" reservoirs?
I don't see a pattern to when I have good braking or not. I could go the whole day & not have an issue, or I could have good brakes on on stop & not on the next. I have also noticed that sometimes I have poor braking, then it feels like the boost kicks in & the brakes start acting normally.
I know this is asking a lot for a diagnosis from just a post in a forum. My thing is that I can replace a brake booster (firewall mounted), but have no experience with these other chambers. If these other chambers are indeed like a big truck brake booster, I know they have powerful springs in them & you really need to know what you are doing to rebuild/repair them without taking your head off.
Any help or thoughts would be appreciated.

Can't help with your mystery canisters, but your description of the brakes being weak at first, then getting stronger as you depress the pedal, sounds like hydro-boost kicking in. Check to see whether your brakes are equipped with hydro-boost - it runs off the power steering pump, & your intermittent problem could be some malfunction there. Low fluid? Slipping belt? Or?

I don't have a hydro-boost system. they don't always start out weak, then get better. sometimes they are just hard like you'd get when the brake booster goes out in your car or truck. Sometimes they are normal. Then sometimes they start out being hard then you feel something "give" & then they start acting normal.
I'm wondering if those vacuum canisters under the coach have valving in them that can be sticking?

I would check brake calipers and/or bridges. Sounds like one is sticking in the bore. Do not jump to conclusions, that can get very $$$

Thanks Billy.
I think I can probably rule out caliper problems, as I rebuilt both of them summer before last when I did the front brakes. This is definitely related to vacuum. I know there is adequate vacuum being generated by the engine, as there is a gauge on the dash. I've checked it against my hand held & it seems to be fairly accurate. I think I'm going to look at the valve on the booster to make sure it's not faulty, then I will do some testing to try to isolate if it might be the main booster.
I'll keep posting back.
I'm surprised there are no big truck mechanics lurking here.

Hey Billy...on a side note...did you go to the international food fest this year? LeAnna & I went a few years back & had the best time. You folks down that way are the friendliest people! We were wanting to go this year but weren't able. *sigh* Maybe next year.

There are Big Truck mechanics here, but the P30 chassis is kind of like a light truck with a gland condition.

It's hard to say what the two canisters on the frame rails are, but likely the only accumulator is spherical and somewhere in the engine compartment. Hydroboost on a gaso engine would be rare.

Two suspects come to mind, the cruise control dump valve (and the tubing to the cruise control) and the brake booster assembly. Try blocking off the tubing to the cruise control and see if that cures the issue; if so, the booster is the prime suspect. Somtimes the diaphragm will have a tear with a flap that sometimes closes, sometimes not.

quote:

Originally posted by Rusty:
Hydroboost on a gaso engine would be rare.

The larger P30s had hydroboost. Mine does. So did the larger GM pickups. The hydroboost has an accumulator, but it is attached to the master cylinder.

Mine (with Hydroboost) has no accumulator.

I had a brainpfart, and meant to make the statement that "Hydroboost on smaller gaso rigs would be rare"; my 24' Fleetwood Class C G30HD had Hydroboost. Naturally, the P/S pump belt crapped when I drove it home after buying, so I got to drive 7 miles w/o P/S or power brakes...

As an aside, my LWB 1-T Ford diesel van, with a higher GVW than the 24' Fleetwood, has vacuum assist (belt-driven pump).

quote:

Originally posted by Mark Estep:
I'm surprised there are no big truck mechanics lurking here.

What???

quote:

Originally posted by Rusty:
...but the P30 chassis is kind of like a light truck with a gland condition.

Exactly... As much as I would like to comment, I have no idea what it is that you're describing.

Now I'm sure, if I could see it, I could give you an answer in 30 seconds.

quote:

quote:

...but the P30 chassis is kind of like a light truck with a gland condition.

Exactly... As much as I would like to comment, I have no idea what it is that you're describing.

Meaning it may be large, but the drivetrain and accessories are related to light trucks rather than medium and heavier.

Bill,
It's raining here right now & looks like it will continue a lot this week, but I'll post some pix of the "mystery cans" at the first opportunity.

OK!....So I'm a slug....I admit it. I'm sorry I never got back here with anything further. I've got too many irons and not enough fire (if you know what I mean).

Well, I _FINALLY_ figured out what the trouble with the brakes was. I traced the hard line back to the vacuum source and found the culprit to be my vacuum booster check valve. I determined that a weak spring was allowing it to be sucked shut sometimes and ergo, no vacuum assist to the boosters. It would work sometimes because the valve plunger would get cocked to the side and not block the port completely and other times it did. It explains why I would have trouble out of my cruise control as well. The cruise gets its vacuum from the booster check valve also.

Since I have original documentation of everything for this vehicle, I thought "Well, it's all downhill from here." And how prophetic that turned out to be! Armed with a GM part number, I went from parts house to truck parts house & finally to GM, but no joy. Even trying to buy component pieces of the valve to build my own was no go. I was told it was a discontinued piece in 1998 by the guy at GM.

As a last resort, I decided to substitute a check valve from something else & just plumb it in with hose. The original just sat on the engine and connected to the hard line with hose. I was amazed to find that booster check valves were almost never sold alone. "Comes with the booster." was a phrase I heard too many times.

I called a guy at NAPA & was told that they had a few check valves. I rushed over to take a look and the first one he pulled up on his computer catalog was...

this

Yep....The exact one I had been searching for! He had to get it from the warehouse the next day, but that night there was much singing in Whoville!! And the best part was that it was only a bit over $10 with tax!

I guess I should have went to NAPA first, looking back at it. They are the only place I can get things like wiper blades for my Barth.

Who knew?

Thanks for everybody's input. I really appreciate it. (I'm trying to butter ya up to read my other post about my sway bar....is it working?)

Maybe in about 3 months.

quote:

Originally posted by Mark Estep:

I called a guy at NAPA & was told that they had a few check valves. I rushed over to take a look and the first one he pulled up on his computer catalog was...

this

Linkee no workee.

Hi Mark and friends,

I'm having similar braking difficulties with a 1972 22ft Barth equipped with the same mystery vacuum canisters. The link to this check valve is no longer functioning. I disassembled the first of the two check valves and it appears to be working, though, I was surprised to see a little bit of fluid inside. Not sure if it was brake fluid, might have been in part, but it was mostly odorless.