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Hydraulic Tag Axle Brakes

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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


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