disk / drum power brakes combo valve issue

I used the search function and read through tons of the existing threads regarding power disk / drum brakes, but could not find the solution to my problem. If there is an existing thread that already details a solution to this issue, please feel free to post the link.

My 53 Merc has front disk brakes as well as the Jamco power brake ***embly kit (8" booster, dual chamber m/c, combo valve). The only weird thing about the current installation is that the front brakes are fed from only one of the 2 available combo valve outputs (then split with a T fitting) and the other combo valve output intended for the front is used for the brake light switch.

The front brakes hold so much pressure that I only get 1/4" to 1/2" of pedal travel. I hoped that after a break in period, perhaps things would settle in and the brakes operate correctly, that is not the case. After a small amount of around town driving, the front brakes are close to fully locked up. The brake light stays on even without the pedal being applied, which indicates that at rest, the front brakes are receiving pressure.

Do I have a bad combo valve? Do I have the wrong combo valve? Or is it necessary to connect drivers front brake to one outlet of the valve / p*** front to the other outlet of the valve (possibly using a t fitting to plumb in the brake light switch)?

 

Go to the ECI Brakes web site click on brake facts read metering valve info. Hope this helps.
PS they are very good people to work with.
Good luck

 

Thanks for the link. Good information ( I now understand why I need to add some 10psi valves to my dual rez non-power drum F100). As far as my current situation with the Merc, looks like I do have what is described on the ECI site as "a metering/hold-off valve in the system either as a stand alone valve or as part of a factory valve" with the GM combo valve that is currently in place. What I need to figure out is (1) is the location of the brake light switch causing the problem, or (2) Do I have a bad combo valve, or (3) should I be looking for a different cause of the constant pressure to the front brakes?

 

I'm having a similar problem with my 58 ranchero, after awhile the fronts are dragging. I found a couple of problems, the top pivot where the linkage pushes the rod into the booster had too big of a cotter pin, it rotated and held against the firewall, holding pressure. Another suggestion was the pin between the master and the booster should have a small amount of play, if it is to long, like mine it applies brake pressure always. I drove the car and it was good untill I hit the brakes hard, so I shortened the rod pushing into the booster, waiting to drive it. Mark

 

Holding pressure after getting off the pedal is sometimes due to not having a proper gap between the brake pedal rod and the piston in the MC. 1/8" play is what I use.

Grab the pedal with your hand and check by feeling for a bit of play before the rod that goes into the MC hits the piston. If you can't feel any play dig deeper.

 

Steve,

It's common to plug one of the outlets on the combo valve to the front brakes, and just use one outlet for the front and T the line to the front calipers.

You didn't mention residual valves, so I ***ume you have the MC located on the firewall. If the MC is on the frame and you are using residual valves, the brake light switch needs to be close to the MC before one of the residual valves.

I would remove the brake light switch and plug the outlet and see if anything changes. I would plumb the brake light switch directly into the line close to the MC.

Good luck,
X

 

Thanks for the replies. I have checked everything out, and it looks like I have good free play with the pedal rod, the calipers are mounted with correct alignment (they do release once I remove pressure via opening the bleeder valve).

**zz - I agree, I would have (and will, while I am working on all this) plumbed the brake light switch into a T fitting between the MC and the combo (residual / metering) valve, rather than using one of the front outputs of the combo valve. For reasons that I can't really elaborate on, a few different people did some work on my car, and I am still in the process of undoing / redoing much of that "favor". Unfortunately, the brake light switch is staying on because the system is not releasing the hydraulic pressure.

From what I can tell, either the mc or combo valve is sticking and not letting the pressure release. I am going to throw a new valve and mc at this and see what happens..