Very high brake pressure

[Winnebago73]

I have a 1973 Winnebago, bought it a year ago. My brakes when cold work well, after 10/15 miles down the road they become so pumped up. At this point I can't push the brake pedal down at all.
What can I do to fix this?
Kurt

[DaveVA78Chieftain]

Do you have drum or disc front brakes?

Dave

PS: Mark will most likely move this thread over to the Dodge Chassis forum as that is where it belongs.

[Winnebago73]

I have drums all around. Tried bleeding still same outcome. I am baffled?

[brians69d24]

Could be a booster problem.

[Winnebago73]

Is there a better way, to find out we're I can pin point this area of brake failure ?

[DaveVA78Chieftain]

If you do not have the MH Service Manual, then either buy one from the Store here or you can use my web site where I have posted the information you need to work through this.  You have the 1971-73 M300 (Late) model Configuration.
Note: My site only has some areas of info, not the entire MH Service manual.  I was simply trying to remove some of the mystery of the different brake systems Dodge used on the MH chassis by organizing it to the style brake system installed for a given configuration.

On my site (http://dave78chieftain.zxq.net/), select Brake Info (http://dave78chieftain.zxq.net/DodgeRBrake.html), then 1971-73 M300 (Late) model (http://dave78chieftain.zxq.net/M300_71-Late.html).  There are links to all the related information for your configuration from there.

This could be a Bendix Master-Vac booster "Dragging Brakes" issue (http://dave78chieftain.zxq.net/MasterVac_brakes.html) and requires booster replacement;

or

a Bendix Dual System Master Cylinder Residual valve related issue: http://dave78chieftain.zxq.net/MCBendixDual.html

From "http://www.thebrakeman.com/valvetechi"
This valve does exactly as its name suggests. It keeps a pre-determined amount of residual pressure in the line after you remove your foot from the brake pedal. This aids in preventing excessive pedal travel as well as insuring consistent height to the pedal. In a drum brake, heavy return springs are present to pull the shoes away from the drums. When not in use, the shoes are pulled back until they rest on a centering pin, usually located at the 12:00, or top position, on the backing plate also holding the wheel cylinder. In order to avoid the excessive pedal travel to move enough fluid from the master to activate the shoes, a 10-12 pound residual valve is installed in the line. Since the return springs are stronger than the 12 pound valve, the shoes are pulled away from the drum in spite of the resistance so no brake drag results.

These valves are not used in a disc brake application which is why I asked about drum vs disc.

So, in your case, the valve could be corroded or the brake shoe springs have weakened to the point that they can no longer pull the shoes back away from the drum (or a broken spring).

Dave

[brians69d24]

Another thought I had: After driving for the 10 minutes or so, stop and feel all the drums. Are any hot? Is the fluid in the master cylinder relatively cool?

[Winnebago73]

I will check that, here's what happens. The RV is cold start and drive for a few miles some further some shorter in distance. If I apply the brakes lets say 6 times  it suddenly becomes so pumped up and hard you can't push breaks anymore. What I have noticed my idel increases at this point, if my idel stays at its normal my breaks are ok. Looking at your brake Manuals, mine is different to those illustrated. But basically follows the pattern. It feels like a vac is needed but can't figure out were it would go. Nothing missing!

[jkilbert]

It sounds like you may even have a vacuum issue. Check the line that runs from the manifold to the booster. You have higher vacuum at idle than when under power. If you have a leak in that line or in the check valve on the booster is bad you'll lose vacuum and end up with a hard pedal.

john

[DaveVA78Chieftain]

By different, do you mean in location or in basic shape?
The main over all page is http://dave78chieftain.zxq.net/M300_71-Late.html
The vacuum lines are the larger ones from the booster (attached to master cylinder) to the engine and the booster to the break air cleaner (usually located around drivers seat)
Dave

[Winnebago73]

Morning Dave, and thanks again for your good advise. I finally sorted the problem out, doing the elimination test throughout the complete brake system. A vacuum line had a leak, because of that I also noticed my rear drivers back brake had rusted in. I would guess not enough fluid could make it back there because of the malfunction of the vacuum?
All I know I couldn't have done it without your help.
Sincerely

Kurt

[DaveVA78Chieftain]

Glad the information helped you to work through the issues.  Rust is rust and binds things up when not used frequently.  The rear wheel was just simply mother natures way of saying pay attention to me now and then!