Braking issues

[Jerry G]

I have an older 1994 Holiday Rambler Endeavor LE, I need Some help figuring out the master cylinder proportioning valve. On front engine cars and trucks the valve sends most of the pressure to the front brakes as this is where most of the weight is. My RV has a rear mounted engine and transmission so most of the weight is on the rear duals and yet the master cylinder, not the original one, is still setup to deliver most of the pressure to the front brakes. This causes the front brakes to lock up under heavy braking. The Rv has the same size rotors and calipers front and back and everything is new. can anyone tell me whether this is correct for a rear engine vehicle, or had some installed the wrong master cylinder in it at some point.
1994 Endeavor
Oshkosh frame
Ford hydro boost disk brakes
Cummins 5.9
Allison transmission

 

[8Muddypaws]

I'm thinking that proportioning has very little to so with static weight distribution and lots to do with the weight transfer to the front wheels under braking.

 

[Jerry G]

I'm thinking that proportioning has very little to so with static weight distribution and lots to do with the weight transfer to the front wheels under braking.

My issue is that the front brakes tend to lockup when you try to stop on wet roads. I had to replace the rotors on the front because they were cracked in several places and the back one were in good condition. My thinking is that if the majority of the weight is up front on a car, but it is mostly sitting on the back of the RV with a diesel pusher.

 

[donn]

Go find an adjustable proportioning valve. That will allow you to play with the F/R balance to suit your wishes.

 

[Jerry G]

I have an older 1994 Holiday Rambler Endeavor LE, I need Some help figuring out the master cylinder proportioning valve. On front engine cars and trucks the valve sends most of the pressure to the front brakes as this is where most of the weight is. My RV has a rear mounted engine and transmission so most of the weight is on the rear duals and yet the master cylinder, not the original one, is still setup to deliver most of the pressure to the front brakes. This causes the front brakes to lock up under heavy braking. The Rv has the same size rotors and calipers front and back and everything is new. can anyone tell me whether this is correct for a rear engine vehicle, or had someone possibly installed the wrong master cylinder in it at some point.
1994 Endeavor
Oshkosh frame
Ford hydro boost disk brakes
Cummins 5.9
Allison transmission

 

[Jerry G]

Go find an adjustable proportioning valve. That will allow you to play with the F/R balance to suit your wishes.

Unfortunately the proportioning valve is built into the master cylinder and there is no way to bypass it. If I can figure out whether it is backward it Can be corrected by re-plumbing the brake lines at the master cylinder.

 

[johnhicks]

I've never once locked up the front brakes on my 1993 HR DP; the brake wizards told me that in a MH the rear brakes do most of the braking even though everything is the same size. I suspect someone stuck in a master cylinder intended for a pickup truck.

You can learn the master cylinder part number from Freightiner with your VIN.

 

[JayArr]

That's not how proportioning valves work. They don't divide the pressure between the two sets of brakes, they regulate the rear.

From Wilwood Brakes website:

The name "proportioning valve" seems to confuse people. The valve is more of a pressure regulator, decreasing how much pressure goes through it by a set amount (once initial pressure reaches a certain point). Reducing the force going to one circuit doesn't increase the force going to the other brake circuit. However, when the valve reduces pressure to one circuit, the proportional relationship between the front and rear braking (front to rear brake bias) changes.

How Does a Proportioning Valve Work? And How Do You Adjust It?

When setting up a street car for performance driving or converting a classic to disc brakes, it is essential to include an adjustable proportioning valve in your brake system. This valve is typically plumbed into the rear brake line or built into the combination valve below the master cylinder...

shop.wilwood.com

It's not like the valve sends 60% to the front and 40% to the rear it's more like the front gets 1200psi and the rear gets 800psi. But only in a stomp on the brakes scenario. In normal braking around town they should get equal pressure, like 600 and 600psi.

I would say that if your front brakes are wearing out too quick then your rears aren't doing their job. Are the self adjusting? Are the cylinders working? Put the rig on stands and a soft press of the brake should stop the rear tires in drive at idle.

The proportioning valve should not come into play at all until it is a sudden braking situation where you hit the pedal hard.

 

[Jerry G]

I've never once locked up the front brakes on my 1993 HR DP; the brake wizards told me that in a MH the rear brakes do most of the braking even though everything is the same size. I suspect someone stuck in a master cylinder intended for a pickup truck.

You can learn the master cylinder part number from Freightiner with your VIN.

Thanks, that is helpful. Now if you would just crawl around under your and get me the part number off the master cylinder I will look it up or swap the brake lines at the master cylinder. Could you forward the web site for the correct Freightliner site?

 

[Jerry G]

That's not how proportioning valves work. They don't divide the pressure between the two sets of brakes, they regulate the rear.

From Wilwood Brakes website:

How Does a Proportioning Valve Work? And How Do You Adjust It?

When setting up a street car for performance driving or converting a classic to disc brakes, it is essential to include an adjustable proportioning valve in your brake system. This valve is typically plumbed into the rear brake line or built into the combination valve below the master cylinder...

shop.wilwood.com

It's not like the valve sends 60% to the front and 40% to the rear it's more like the front gets 1200psi and the rear gets 800psi. But only in a stomp on the brakes scenario. In normal braking around town they should get equal pressure, like 600 and 600psi.

I would say that if your front brakes are wearing out too quick then your rears aren't doing their job. Are the self adjusting? Are the cylinders working? Put the rig on stands and a soft press of the brake should stop the rear tires in drive at idle.

The proportioning valve should not come into play at all until it is a sudden braking situation where you hit the pedal hard.

I probably could do a better job of explaining, my question is about the built in proportioning valve in my master cylinder. it reduces flow to the rear brakes when the brakes are applied, it is working as it was designed to function. My question is: on a rear engine motorhome should the valve reduce flow to the front brakes instead as the weight is over the rear axle.
It has disk brakes front and back, all new calipers, pads, brake lines and it has been flushed.
I just replaced the master cylinder, so the issue may have been solved. Will take it for a test drive as soon as possible to see if it works properly. Thanks for the Response.

 

[JayArr]

My question is: on a rear engine motorhome should the valve reduce flow to the front brakes instead as the weight is over the rear axle.

No, it doesn't matter where the engine is.

 

[8Muddypaws]

And that takes us back to what I originally said. Sort of.

 

[Gary RV_Wizard]

Just to be clear, we are talking about a rear-engine coach with hydraulic brakes rather than air? I'm thinking this must be an Oshkosh chassis? And the master cylinder is not the original nor an identical replacement?
Brake system parts are available for those at John Deere / Oshkosh Brake Parts | Mill Supply, Inc.

 

[johnhicks]

I believe now you have to call a Freightliner dealer/shop. My chassis docs only say "Bendix," not which Bendix.

 

[House Husband]

That piece displayed with the master cylinder is not a proportioning valve. It is part of a ground switching system that turns on a dash light when the front or rear of the braking system fails. Note the bump to keep the electrical connection on. Proportioning valves are use to balance the braking pressure when disc/drum system are used.
That master cylinder was used on many Ford products.

Richard

 

[JayArr]

I don't know about Fords but on my Cadillac that is installed in the middle of the proportioning valve. I agree that it's for turning on the dash light but the valve piston that actuates it is part of my proportioning valve.

Off topic note: I have four wheel discs and a proportioning valve so it's not 'only' for disc/drum setups.

 

[House Husband]

I don't know about Fords but on my Cadillac that is installed in the middle of the proportioning valve. I agree that it's for turning on the dash light but the valve piston that actuates it is part of my proportioning valve.

Off topic note: I have four wheel discs and a proportioning valve so it's not 'only' for disc/drum setups.

Proportioning valves are NOT part of the master cylinder but are separate devices to control line pressure. The piston, in the master cylinder, that actuates the grounding device for the dash light, is just that and does nothing more. Some of those pistons are spring centered and the light goes out when the brake pedal is released and some are not spring centered and must be manually reset once forced off center.
I speak from years of working professionally as a mechanic.

Richard

 

[youracman]

I don't know about Fords but on my Cadillac that is installed in the middle of the proportioning valve. I agree that it's for turning on the dash light but the valve piston that actuates it is part of my proportioning valve.

Off topic note: I have four wheel discs and a proportioning valve so it's not 'only' for disc/drum setups.

I have discs all-around on my Class C, as well. I have done a brake job on both front and rear axles and the rears were worn the most. That surprised me because Rush Ford Truck Center told me my proportioning valve is 70/30 (70 front/30 rear) and I always thought that meant the front brakes worked a LOT harder than the rears in "normal" operation; that is, until I read up on proportioning valves ......... and mine works like Jay described. Just an aside: At NAPA's suggestion, I installed semi-metallic pads. They told me the rig would stop better but the brakes would be "noisier" ......... and they were right. I was also told there would be more rotor wear.....but I could care less; I have had to make just one emerg stop and it scared the heck out of me........air brakes would be wonderful, me thinks. :^)

Still, the OP surely has a valid concern ..... what with the fronts "locking up" on his Class A. Good luck with your detective work and the fix, Jerry.