J-3 Piper Cub: fixing the brakes

CUB HOME PAGE > BRAKES

Fixing the brakes on a J-3 Cub

The following requet was posted on the Cub Builders mailing list:

"Hi,all. Had the wheels off my J-3 and noticed that I have heavy gouges in the brake drums. Bled the brakes including putting a band clamp around the expander tubes as suggested in a recent Cub Clues [newsletter] to squeeze out any fluid and air. Seems like the pads still stand "proud" a little from the frame in a few areas--the cause for the gouges? If so, is there anything I can do? Or do I possibly have expander tube issues? I [could] hear brake dragging on both sides when spinning the wheel. Also, I might have wheel bearing issues, [since] when I tighten the axle nut sufficiently I get a dragging wheel. Loosen enough to let the wheel spin, and I have a little end-play and a very loose axle nut. Trying to stay with my standard Cub setup but may go with the Grove Aircraft setup if I can't get mine to work. Any suggestions or opinions appreciated. Thanks, Dave"

An answer from Len Buckel:

I think that the grooves in the drum were caused at some time in the past when one of the brake block retaining springs broke. Parts of the spring stayed in the wheel for a while and "turned" the groves in the drums. This is pretty common.

[At this writing,] Robbie Grove (Grove Aircraft) doesn't yet have the STC on his brake for use on a J-3. Some of these kits have been installed on experimental Cubs and on home built "Cubbies". They are much cheaper than Clevelands. The STC that he is working on will allow the use of your old wheel and 8.00 x 4 tire with the drum replaced with his disc and hydraulic cylinder.

I have seen a J-3 on its nose using stock brakes. Many years ago, at an open house at our airport there was a competition between the old trainer (the J-3) and the current trainer (a Cessna 150) to see which could get off faster and land shorter. The C90 powered J-3 won both, although the J-3 on its nose stopped shorter than the owner would have liked.

The brake blocks that had to be sanded down may have been oversized blocks. Univair had a supply of these [oversized blocks] for a long time. I witnessed this same problem when some of these were installed.

The wheel bearings (if good) should have no effect on the turning of the wheel unless they are adjusted too tight. This will ruin the bearings after a time. (Loose is better)

There were two different brake fluids used in the Cub brake systems. Piper Service Memo No. 4 (should be available from the Cub Club) [says] that a vegetable or castor oil base fluid was used in the earlier systems. The Service Memo indicates that the red fluid was used "approximately from 1942 on." The memo tells how to identify the different type tubes that are marked with different color stripes. I think that many of these stripes have disappeared by now. If you can't identify the type of tube, I would use silicone fluid as I have been told that it is compatible with both types.

Adjusting the brakes

I was told by an "Old Mechanic" that the best way to adjust bladder type brakes is:

1. Completely fill brake system through the master cylinder fill plug. Clear to the top. No air in system.

2. Fill a pressure type oil can with brake fluid.

3. Raise and block the wheel so that it is clear of the ground and will spin freely.

4. Remove the screw from the bleeder valve at the brake.

5. Place a layer or two of rag or shop towel between the tip of the oil can and the bleed screw hole. This will help seal between the two. Pump oiler until all air is out of oiler spout and bleed screw hole. Open bleed screw and force brake fluid into expander tube while turning wheel. Close bleed screw when wheel stops turning.

6. Set oiler aside. Crack bleed screw and let a little bit of fluid out. Continue draining small amounts of fluid until the wheel turns free. Continue bleeding small amounts until the brake pedal has about 3/4 inch of travel before it becomes hard.

This procedure moves the brake blocks close to the drum. Then it doesn't require excessive pedal movement for maximum braking effect. This is also better for the master cylinder diaphragm as it doesn't have to be stretched as much. I believe that most diaphragm failures are caused by excessive stretching to get the brake blocks out to the drum.

Granted, the parts for these brakes are expensive but in 29 years and almost 2500 hours operating my J3, I have had [only] two problems.

And a Cub looks better with 8.00 x 4 tires on it!

Len

Question? Comment? Newsletter? Send me an email. Blue skies! -- Dan Ford

Return to the front page.

Other websites: the Warbird's Forum | Daniel Ford's books | Facebook | Sail Alaska's coast | Reading Proust