1/25 scale Transtar 4300: Drive axle brake chambers

Brake chambers on the Transtar 4300 kit are a good example of how kit parts provide a starting point that, with a little additional work, can become a small model in their own right.

0:19 Basic shape and appearance of assembled kit parts is good overall, with one item that looks out of place which is the long pin projecting from end of brake chamber cylinder.

0:35 Although not on a Transtar 4300, this brake chamber is on an International (S2600) with similar Hendrickson suspension. Instead of a pin sticking out end of brake chamber there is a rubber cover.

0:58 This image shows basic construction of a "piggyback" style brake chamber, with service brake section closest to slack adjuster and the parking brake section closer to right side of image. Parking brakes are applied with a strong spring and released by compressing that spring with air. If the parking brakes must be released and air system is not operational, a "caging bolt" is used to manually compress parking brake spring. Picture shows a caging bolt in use and may be what Ertl was replicating when they tooled the kit.

1:38 Real brake chamber shows similar appearance to kit part.

2:05 Rubber cover over hole for caging bolt.

2:22 Cover removed showing caging bolt access hole.

2:30 Lugs on end of caging bolt engage spring...

2:41...then a nut is threaded onto bolt to compress spring. Caging bolt is only used when brake chamber is being disassembled for repair, or if parking brakes must be manually released due to truck air system not working.

2:54 Basic mounting bracket detail is provided, although on most trucks the pin projecting to right of bracket (representing torque tube through which the rod on which S-cam is mounted) is a round pipe, not tapered like kit part.

3:15 Areas highlighted in red will be removed or reshaped.

3:23 Stock kit brake chamber with the "upgrade" parts: .060" x .060" square styrene strip for modifying mounting bracket, .040" x .125" and .030" x .030" for slack adjuster, .030" x .060" for mounting bracket to cylinder, .030" rod for bolts and pushrod, .010" disks for slack adjuster and caging bolt cover, ) scale freight car turnbuckle cut in half for pushrod clevis, and 1/16" aluminum tube for brake hose attachment fittings. A completed brake chamber is also shown for comparison.

Attach .060" x .060" styrene strip to edge of brake chamber mounting bracket. This gets brake chamber further away from axle, creating space for slack adjuster and pushrod detail. .060" is wider than kit part, and is sanded flush with bracket after glue has cured.

4:05 Added material sanded to shape and blended in.

4:20 Real brake chamber mounting brackets have an "L" shape that chamber bolts to. This is simulated with .030" x .060" styrene strip glued in place on face of brake chamber against molded-in mounting bracket...

4:35...and drilled in the three places marked for .030" styrene rod.

4:48 Slack adjuster is .040" x .125" styrene strip cut approximately .170" long and corners rounded. .030" x .030" approx. .100" long is glued to bottom, flush with one side. Styrene disk approximately 3/32" diameter completes slack adjuster.

5:19 Tichy Train Group # 2004 turnbuckles, cut in half, make great pushrod clevises. They are sized to fit .030" rod and are a good example of how railroad car sized parts in 1:48 scale can become truck sized parts in 1/25.

5:49 They also fit the .030" square slack adjuster bottom piece nicely. Glue pushrod and slack adjuster together first as a subassembly, keeping the two parts at an approximately 90 degree angle.

6:04 Now glue in place on side of mounting bracket with pushrod inserted into centre hole in brake chamber. The other two holes have .030" rod inserted, projecting approx. .030", to simulate mounting bolts. Two pieces of 1/16" aluminum tube are glued into holes drilled in brake chamber body to represent fittings where brake hoses attach.

6:22 Styrene disk approx. 1/8" diameter simulated rubber cover over caging bolt hole. This completes brake chamber itself.

6:52 Remove molded in round holes on axles.

7:36 Glue brake chamber mounting bracket to suspension mount to area highlighted in red.

7:55 Add 3/32" styrene tube between mounting bracket and backing plate to complete the installation.

8:12 Rear drive axle brake chambers from kit are a style I have not seen before, most trucks I have seen have "piggyback" brake chambers on both drive axles.

8:35 I used identical Ertl chambers from another kit (S2500) and found that they are all molded the same, instead of being left and right hand like real brake chambers. This can be corrected by cutting off and reinstalling mounting bracket.

9:15 A simple part, but a worthwhile detail upgrade. Air lines and valves will be covered in next instalment.

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