Hard Brake Pedal - Dual Bendix Hydro-Vac

[Whalerho]

What started off as a straight forward brake service has grown into a small nightmare.


After receiving my 1972 Winnebago Chieftain D-22C (5 lug, dual Bendix Hydro-vac boosters) last year, I started a ground up project to get it reliable and liveable.  Obviously the brakes are important to any build so do the normal maintenance on all four wheels.  The pedal had a bit of travel so I took the drum brakes off and noted several of the springs were broken and a couple of the wheel cylinders were leaking.  I ordered new sets of shoes, wheel cylinders and complete spring kits and installed without problems.  The only issue seemed to be a very hard pedal which barely kept the RV from moving when at a stop. 


Did the checks and found 1 hydro-vac booster to be faulty so I replaced both and had all the lines pressure bled to remove any air but still hard pedal.  The only original item left was the master cylinder so I changed that and pressure bled again.  Still hard pedal.


My mechanic is out of options now on what else could be the cause of the hard pedal.  Everything has been replaced (except for the brake lines, these if good vacuum at the boosters, all lines pressure bled and brakes adjusted.   I know drum brakes do not have the same stopping power as disc brakes but mince can barely keep the RV held stopped while idling.


I hope there are some folks out there that can give me some ideas on what else to check/replace.


Thanks in advance

[Oz]

Wow, that really is a tough scenario. 

Take a look at this info in Dave's Place.
On the main page, you'll see the options with >> to the right of each, which gives you more sub options.

Start with the.Dodge chassis option and work your way to the info on your specific brake system.

These are very, very thorough write ups, invaluable in helping with brake issues and correct part numbers.

http://dave78chieftain.com/

[Rickf1985]

Hard pedal would tell me that the assist is not working, are you getting vacuum to the boosters? Are the one way valves at the boosters working? I am not real familiar with that particular system but there has to be vacuum to make it work. Dave is the expert on those so hopefully he will chime in soon.

[DaveVA78Chieftain]

Ken has been in touch with me about this for a few weeks.  At this point, I feel they need to use a brake pressure test kit to determine where the low pressure points are.  With this system the master cylinder acts more like an extension of the brake pedal rod that acts on the vacuum booster.  The slave cylinder at each booster controls the action of the vacuum booster as well as applies the pressure increase to the wheel cylinders.  Because of this, the system must be bleed at the master cylinder, then each booster slave cylinder, then at each wheel cylinder.

Notes:
Bleeding The Hydraulic System (With Vacuum Hydraulic Boosters)
Before bleeding hydraulic system on chassis equipped with vacuum brake boosters, the vacuum must be completely eliminated from the booster system. Disconnect manifold tube at booster side of manifold check valve (engine stopped).

CAUTION: When air pressure brake bleeding equipment is used, do not apply more than 25 to 35 pounds of pressure.

A piston stop is provided in the slave cylinder to prevent damage to the return spring while bleeding the brakes. This damage can occur only when the brakes are bled with vacuum present in the booster system.


PS: The mechanic never called me.

[Rickf1985]

Ken, You have Dave helping you. I would follow what he has to offer to the T, He knows what he is talking about with these systems. What I can tell you is if you go out on the net looking for more advice you will find it, tons of it and in the end you will find that 99.9% of it was simply guesswork and each person wanted their guess to be the winner. All this does is causes you to spend more money throwing parts at it instead of proper diagnosis and it also ties up everyones time trying a bunch of stuff offered by people that have probably never even seen one of these systems.

[Froggy1936]

I concur W Rick , Stay with the experienced problem solvers,   The I tried it and it works were just lucky not right ! i?? Bad information causes bad results and lots of wasted time !   Frank

[mytoolman]

Hello...have you had any progress here? I was told when I got my RV by my friend the old owner who never could find the time to do anything more than move it onto the place it sat for 6 years that the brakes stopped but the pedal was hard. I just am in the throws of finishing the rear brakes up where I found non leaking original cylinders with just enough pitting to have me replace them and wonder if that was necessary. I experienced a few landmine experiences in doing so that I have with great effort corrected here that I am willing to share so no one else falls into the quick sand and has to struggle to escape....its best to private message me for phone contact....

[Whalerho]

Update:  Problem finally resolved.  Apparently there was a crack in the vacuum fitting on the intake manifold that was intermittent.  When the transmission was in neutral, the vacuum was good and all the boosters worked well.  However, in Drive position, the small crack reduced the vacuum enough to create the hard pedal situation.   Now that the all is fixed and stopping well, I'm glad I went through the entire brake system and replaced everything from the Master to the Brake shoes.  Should last me a few more year....I hope.

Thanks to everyone for the help

[Oz]

Glad you finally tracked it down! That was a tough one.