Hydraulic Tag Axle Brakes

Started by DaveVA78Chieftain, March 17, 2014, 12:11 AM

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Hydraulic Tag Axles are used on many of the longer RV's.  They can be used on the Chevrolet/GMC P30, Ford 53, and John Deere/Oshkosh/Freightliner Motorhome chassis' amoung others.  The Coach Manufacturer was the one who decided and installed a given tag axle system, not the chassis Mfg.  Most add-on tag axle systems are from MOR-ryde Corporation.

MOR-ryde Legacy Owners Manual

MOR-ryde Legacy Service Manual

The tag axle braking system is a secondary brake system with the original chassis manufactures Hydroboost brake system being the primary braking system.

When the operator applies the primary system brakes, the Sycronyzer Relay Valve produces a vacuum signal that is applied to the tag axle vacuum chamber/master cylinder assembly producing a corresponding hydraulic pressure in the tag axle brake system.  From that point on it is a typical Hydro-vac Drum Brake system.

I have seen several references to people working on Tag axle brakes on the web but it seems most cannot find parts for them. The original Mfg of the brake system (Master Brake Inc.) is no longer in business. Here is some information I located that might help someone in the future:

Midland Vacuum Booster/Master Cylinder  (Bottom of pdf page 4 [catalog page 6]) from BEPCO Products

Midland Vacuum Booster/Master Cylinder (bottom of pdf page 100 [catalog page 2-24])
In the same pdf file you will find the OEM Master Brake Booster/Master Cylinder AND the sycronization valve on pdf page 104 (catalog page 2-28). That pdf is from is from Precision Rebuilders.

While I do not know if the products from this manufacture are compatable with the older systems you could at least upgrade a hydraulic system to their product if need be: BluDot Vacuum/Hydraulic Parts.  Be sure and look at their other pages because they sell both systems and parts for their systems. Their systems are the ones pictured in the first pdf above. You would only need the VB-800 booster (750psi) for a tag axle system. Anything more would to high of a PSI value for the original tag implementation.

Older tag axle systems typically use Dextor axles and brakes. They were produced in both electric and hydrualic brake versions. Electric is controlled by an electric brake controller. There were 2 versions of hydraulic brakes. One has a bolt on wheel cylinder while the other has the wheel cylinder welded to the backing plate. Refer to the Dexter web site for further information. Dextor Axle Parts

Credit: Many of the pictures I post here I found at Photobucket - vicNmin library from his Holiday Rambler.

Note: A Ford F53 chassis implementation is basically the same as described here except that they use a Hydromax booster instead of a hydroboost booster for the main chassis. The secondary tag system would be the same.

A Sychronizer valve

Sychronizer Valve Disassembled
The short tower section at the top is the hydraulic cylinder section (brake hose is attached to it in this picture). The bleeder zert fitting is for bleeding this portion of the main (hydroboost/hydroma) chassis system. The large funnel section below that is used to control the vacuum signal to the Tag booster. The lower half is the atmospheric air section to the other side of the booster.
The small black "U Cup" seal in the above picture is what fails resulting in brake fluid being sucked into the engine vacuum. Should be able to find it through a hydrulic supply house.

Tag Booster and Master Cylinder

Tag Booster Bladder valve that engine vacuum acts on

Alternate Tag Axle Braking Setup

Firewall Mounted Booster, Master Cylinder and Sycronyzer  Valve

Master Cylinder - 1950 Ford F3 (3/4 ton)
New: NMC M4572
Rebuilt: NMC P4572



I had to have my Mor-Ryde suspension replaced on my 1988 Fleetwood Limited two years ago. Well this weekend I decided to go underneath the old girl and take a look at the tag axle brake master-cylinder/booster assembly and check the fluid. To my surprise when the suspension was redone the hydraulic line from the main system to the tag system components now goes directly to the tag axle brake line. The shop that did the work didn't say anything about having done this so I was surprised. Don't think it is a problem because I have driven it many miles since the work and everything has worked fine. Have you heard of anybody else having this change done to their system?



John Deere RV-1000 chassis normally has 4 wheel disc brakes.  Does yours?
Is the tag axle drum or disc brakes?


Four wheel disc brakes and the tag axle is drum brakes. Have to wonder why Fleetwood wouldn't have connected the brake hydraulic systems when they built the coach. Sure simplifies maintenance by not having to crawl underneath to check brake fluid level. I already have an issue with checking the disc brake system master cylinder fluid level because of master cylinder location.


Doing some research so I can properly answer your question


You may think the system is working well but I am not sure it is.

First concern: 
The original Chevy disc brake master cylinder was designed to provide a given volume of brake fluid at a given pressure to the separate front and rear systems. The hydroboost unit ONLY increases the amount of foot pedal force applied to the master cylinder piston.  It does not increase the volume of fluid supplied by the master cylinder.  You can only increase the volume of the master cylinder by increasing the pedal travel (piston travel).
The original tag axle drum brake system was designed to have minimal effect on the original disc brake system.  The very small piston area of the Sychronizer valve only increased the volume of fluid in the rear brake system by a small (negligible) amount.  The Sychronizer valve varied the vacuum signal to the tag brake booster.  The tag brake booster amplified this signal and transferred the energy to the tag brake master cylinder.
By eliminating the tag booster and master cylinder, you have increased the volume of fluid required in the rear brake system.  If the master cylinder size was not increased or the pedal ratio changed, this means the brake pedal must travel farther to transfer enough fluid down the line to compensate for the increased volume incurred by the additional drum brake wheel cylinders.  Because the Master Cylinder actually contains 2 pistons (front and rear brake circuit) this increase in piston travel is ALSO applied to the front brake master cylinder piston.  That ALSO means there is MORE force being applied to the front caliper and rotor.  So what you may think is OK may actually be a result of the front brakes being asked to work harder than they were designed for.

Second Concern:
When front disc and rear drum brakes are used, a residual valve is required in order to maintain light hydraulic pressure in the rear wheel drum brake hydraulic system to keep the wheel cylinder piston cups sealed against the piston bore during "off brake" periods.   Was a residual valve included in your system redesign to account for this disc vs drum brake system design difference?   Would be located in the drum brake fluid line.

Third Concern:
A proportional valve is normally installed to holds off hydraulic pressure to the front disc brakes  to allow the rear drum brake shoes to overcome the return springs and begin to contact the drums. This feature helps prevent locking the front brakes on icy surfaces under light braking conditions.  Same principle would apply to drive axle vs tag axle.

All these concerns were designed into the tag brake system so everything worked correctly together.




Thanks for the info. I will have to look into this further with the shop that did the suspension work.




I contacted Mor-Ryde and they said that the current setup will work fine. The reason the shop made the change to the system was because of some parts for the brake system not being available anymore. I asked about the proportion valve need, disc versus drum, etc., and Mor-Ryde said the system will work fine as is. Thought I would let you know ASAP. I wonder if anybody else has had this change made as well?



I would keep an eye on front disc pad wear then


I have been following this thread with interest because many, many years ago I was doing disc brake conversions on the rear of 4x4 trucks. This was LONG before they came that way from the factory, some GM cars had them and they were the calipers I was using. The problem I was having was that I could not find a master cylinder that was a good fit for the smallish rear calipers and the sometimes huge front calipers one one ton front ends so I would use the regular drum rear brake master cylinder except I would remove the residual pressure valve from them and remove the proportioning valve. This worked quite well and the balance was surprisingly good up to about 38" tires. A lot better than the mud filled drums for sure. But back to this situation. I had a lot of trucks come into my shop where people had done the conversions and then broke the rear in half jumping the truck and would bring in a drum brake rear and just have me put it in to get back on the road quickly. So what you ended up with was a master cylinder with no proportioning valve or residual pressure valve together with front disc brakes and rear drums. No way it should work but they did. Pedal travel was generally long if the rear shoes were not kept close but it worked.

In this case here you have the main system which is four wheel discs and they are going to be doing the majority of the work. Then you have the tag which is going to get some of the work. The shoes are going to have to expand out to the drums before the discs are going to build pressure and work. This will cause an increase in pedal travel which he is probably used to and does not notice. The wheel cylinders in the tag are probably small enough so that when the pressure needed to apply the discs normally that the tag is not locking up. That is going to be more by luck than design unless Mor-Ryde changed the wheel cylinders. What this all boils down to is that the standard discs are going to work as normal, the tag may or may not be working at it's normal rate. Could be more or could be less. Are you sure it is not locking up on wet roads?


I have driven the MH on a lot of wet roads and have not had a problem with the tag locking up. Also, if the brake pedal travel has changed I haven't noticed that at all. Stopping seems the same as before the change. The only thing I will say is that I can't imagine how hard it would be to stop without the hydroboost.


I can tell you that stopping without the hydroboost will be both feet and a butt full of seat cushion! Disc brakes take a lot of force to work so you need a lot of leg to stop them without power. I remember when they first started putting disc brakes on Chevy pick ups and they were not power. Fronts only with drums in the rear and they were a bear to push. The next year they came out with Power Disc brakes with the neat little metal circle on the brake pedal.


Dave - you are my hero!!  I have a 1990 Winnebago Superchief DXL on a John Deere chassis.  My mechanic couldn't find the spring on the very rear axel.  He looked all over locally for the spring and called me to see if I could find it.  Then I came across your original posting regarding the older Tag axels usually being Dexter.  After some more research on the Dexter website I finally located the correct spring and was able to order it.  Thank you so much for sharing your knowledge.  Very much appreciated and spot on with the info.  Who would have guessed that the 2nd rear axel was an add-on made by a third company which is why it wasn't listed under John Deere chassis parts or the Winnebago parts?!?!


Your welcome.  Sometimes it is more about knowing how to search for the information.  I then just put it in a format others can use.  Sometimes, there simply is no information posted on the net.


How's this diagram look for tag breaks setup with an adjustable brake bias proportioning valve on the front and a 2psi residual pressure check vale after the split for the tag?

Or just hook the rears up to the tag and go?


This thread is very helpful, thank you for posting the information. The brakes on my Mor-Ryde tag axle were also disabled by previous owner cutting and plugging the vacuum line to the synchronizer. 

This would be a new option for someone wanting to get their hydraulic brake tag axle working again, an electric over hydraulic brake actuator from Dexter, it's ~$800 online, plus the cost of an electric brake controller if you don't have one already. 


Has anyone else simply plumbed the tag axle drum brakes to the rear disc brake lines to get their tag axle brakes working again?