Lost brakes twice while driving: Vacuum problem?

[drummerboy]

Sent: 8/29/2003

Hi Everyone!  Last night I was delivering our 1972 Indian to it's Labor Day destination to beat traffic.  When I started down a steep hill in our town, I knew I was in trouble.  I slowed down to about 10 mph alright, but I wasn't stopping no way no how.  I had to slam it into park to avoid rear-ending traffic.  Ouch!
This happened previously, but that was from 75mph to an interstate off ramp.  I figured the runway just wasn't long enough.  We added fluid to the rear tank on the master cylinder and bled the brakes.  Everything was fine until the steep hill.  I'm not very familiar with these old braking systems, and neither are any local mechanics.  Does this sound like the master cylinder or is this related the brake boosters I read so much about?  When I got to about 10 mph, the pedal would go no further, even though not to the floor.  The engine even seemed to race when the pedal was fully depressed.  Vacuum? I don't have to bring her home for a while, so any ideas are greatly appreciated.  Thanks much in advance!
Jeff

[DaveVA78Chieftain]

Sent: 8/29/2003

Jeff,
This is one of those troubles that you just have to work through systematically.  The problem could be in the front or rear circuit.  It also depends on what chassis you have (M300, M375) and the type of brake circuit you have.  Some models had just 1 brake booster at the master cylinder.  Others had 2 boosters mounted on the frame rail.  If you have a 71 M375 chassis (pre March 71), it is possible you could have a single reservoir master cylinder.  In other words, need more info about what you have to help properly.

Dave

[MSN Member]

From: Dave-Pam1
Sent: 8/29/2003

Jeff,
Look for fluid leaking out of the wheel cylinders also. It is going somewhere and should be visible where its leaking from.

[denisondc]

Sent: 8/29/2003

drummerboy: I couldn't tell if you lost braking effect itself, or if the pedal effort was too high, relative to the deceleration that resulted. I have tried to stop mine on gentle downgrades with the engine shut off -- and pushing as hard as I could it was still scary. I believe yours is a 72 and longer than 21 ft, so it probably has the dual boosters halfway back inside the frame rail - tell me if I am wrong. This would be the same system mine has. I can think of 5 reasons for your symptoms.
1. The rubber bladder in the large diameter part of the boosters could be split, and when you put on the brakes, atmospheric air is flowing into both sides of the bladder, instead of only the back side. The air pressure differential is therefore less, and so is the braking boost you get. This would also allow some air to flow into the intake manifold, and to the extent the engine didn’t misfire from being lean, it would speed up some. This could also happen if the controlling air valves on the booster were defective or sticky.
2. Both boosters need a vacuum source, so there is a hose from the intake manifold to one booster, with a connection to the other booster. There also should be a hose from that inlet air filter - under the drivers left hip - running back to the boosters. The -Tee- fitting or connection hose taking that air to the other booster might be damaged.
3. Your boosters might have a lot of dusty muck in them - a mixture of a little brake fluid and the dust that that could come in each time you stepped on the brake, if the inlet air filter or hose wasn't intact, over the last 30 years.
Unless you had to put a lot of fluid into one side of the reservoir I would not think lack of fluid was the culprit. However -
4. The braking system is double, each side working 4 of the 8 brakes shoes on the rig. If one side of your braking system was out of fluid, the pedal would go about halfway down before the other part of the master cylinder would be operated, and you would of course have lost half of your braking power anyway. The thing would still stop, but it would want to pull either left or right, since only one side of the front axle brakes would be working. And if the half of your brake system that remained wasn’t in good trim, the booster working well, the brake shoes all adjusted, etc. then it would be a grim thing to stop on a downhill.
5. The self adjusters inside the brake drums are possibly stuck, and haven't kept up with advancing your brake shoes. This makes the boosters need to travel further, and whatever muck is in the booster air chamber becomes a limiting factor on booster piston travel. Have any of you folks ever opened up an old truck booster and found it clean and dry inside?
If it is losing fluid it will eventually show up somewhere as a damp spot, or slimy streaks down the tire sidewall, and you might want to sit pushing on the brake pedal or use a stick, and give it a few minutes for a leak to show up. Except - if there is a leak in the hydraulic portion of the boosters: To examine that; pull the hose off the intake manifold and see if the hose is dry inside. It should not have any brake fluid in it, or you have a bad bendix hydrovac booster. That vacuum hose is where the brake fluid will go, if there is a leak in either hydraulic assembly of the Bendix hydrovac unit. If you find the hose is oily inside but it isn’t brake fluid - you should replace the hose, and see if that -engine- oil got all the way back to the boosters.
And when you look for leaks, look along the brake lines, fittings and hoses too. Refilling that master cylinder is real fun isn’t it? denison
P.S. Does yours have the little parking brake drum mounted on the end of the transmission? Does it work? I got mine working in order to pass the annual safety inspection. Thats about all I think it is good for.

[drummerboy]

Sent: 9/6/2003 6:14 AM

Thanks much Denison for all the information!  We brought her home Monday and everything worked fine.  I came home a different route of course!  I'll start from your first suggestion and work my way down.  Sorry it took so long for the thanks, but we're roofing our 'non-winnie' home and I haven't been on the computer this week.
Thanks again to all!
Jeff

[Chief Crazy Canoe]

Denisondc-


Fantastic explanation & diagnostic advice.


I'm in the process of refurbishing my M375 and this was great to find.

Interestingly enough - the OP never mentioned about the noise coming from the filter (floor next to driver seat) - as when I depress the pedal i get very little braking effect and a sound of air rushing from the filter.

I know I have poor vacuum line quality as they're all dry rotted.  I'm planning on starting with the vacuum lines and then replace all solid and rubber brake lines as they're so badly deteriorated.

It is a fun time restoring these classics!!