Master Cylinder Tech Tips


Bleeding the Stock Master Cylinder

Master Cylinder Exploded View

Jon Paulsen: "When I had a stock master cylinder on my CJ-3B, I got the cap off with a 1" socket, either on a ratchet handle thru the access plate, or from above, thru the engine compartment using a bunch of extensions, a u-joint and ratchet. I added fluid from above too, as it seems to spill less. I used a funnel with a piece of rubber hose taped to it.

"Since this is a single master cylinder system, if you ever lose fluid anywhere (master or slave cylinder leak or broken hose), you lose your brakes completely. So a few years back, I rebuilt the master cylinder and replaced all four slave cylinders, and all the tubing. I put 11" drums up front while I was at it. Now I flush it out every year to keep it in top shape. I bought a vacuum pump and made a bleeding kit, so the hardest part of the stock system is getting that darned cap off, and the 2nd hardest part is getting the fluid in. A complete vacuum bleeding kit w/pump is available for about $22.00.

Eric Lawson adds: "When bleeding brakes, most people put the bleeder (container) on the ground and run the tubing from the brake cylinder's bleed port to this container. This puts the highest point of the bleed system next to the brake cylinder's bleed port. Since bubbles in brake fluid want to be at the highest point possible, they will "hang out" at the bleed port. To keep these bubbles from being sucked back into the brake cylinder when the brake pedal is released, two people are needed. One to press the brake pedal and another to open and close the bleed valve.

"To get around this problem, I punch a hole in a jar, run the bleed tube through the hole and then tape the tube in place. I also tape a strong magnet to the jar. I can then stick the jar onto anything magnetic that is above the bleed port. As long as the tubing makes a reasonably straight run upward and doesn't have any loops in it, the bubbles will rise AWAY from the bleed port.

"Doing it this way, one person can bleed the brakes, as there isn't any need to open and close the bleeder valve to keep the 'evil bubbles' away."

Waterproofing the Brake System

Jon Paulsen: "One problem with the stock master cylinder is the fact that it is not a sealed system, letting moisture and contamination enter. This is compounded by the fact that the vent is located on the MC cap, down low by the frame. If your Jeep likes to go swimming, water will surely enter the system, which wreaks havock with MC's, brake cylinders, and metal tubing. It also deteriorates the effectiveness of the fluid. This problem is easy to fix with about $10.00 worth of parts. Other benefits include ease of service and larger fluid capacity.

Required parts:

"I used the milky white, stiff plastic tubing made for plumbing. Test tubing by submersion in brake fluid for several days before proceding. Clear vinyl tubing will harden and crack or leak at the fitting. Rubber brake tubing should work well, but you wouldn't be able to view fluid level. Teflon tubing would be perfect, but most expensive and difficult to find locally.

"When I ordered my M38 MC cap from Willys Works (11/97), they only had six left. You may have to find another source. They are steel, and should last forever, so there is no need to order a spare. If they become completely unavailable, you could seal the vents in the stock plastic cap, remove the metal baffle (if it has one), and drill and tap for a fitting.


  1. Replace MC cap with M38 MC cap. If using a barbed fitting, it might be easier to install the fitting in the cap first. Torque her down good, if all goes well, you will not have to remove it ever again.
  2. Install the tubing and adaptor. Route the tubing away from exhaust manifolds and moving parts [i.e. floor pedals]. At this point, route the tubing straight up.
  3. Attach a clean funnel to the end of the tubing. This is a good time to flush and bleed the system. Slowly add enough fluid to fill at least half of the tubing. Check for leaks at the fitting. If using a barbed fitting, you may have to use a hose clamp to get a good seal. Flushing and bleeding is a lot more fun now. Easy to see the fluid level and add fluid. A one man operation with a vacuum pump. If using a vacuum pump, start bleeding with the brake closest to the MC. Attaching the funnel to an overhead rafter makes this easier.
  4. Remove funnel and arch the tubing over the fender to drain out excess fluid. We want enough fluid left in to see the level, but not enough for it to spill out the end. Fender height seems about right.
  5. Wipe any fluid from exterior of tubing. Slip finger cot over the end of the tubing and secure with zip tie. This seals the system, but allows for expansion as the brake pedal is pressed. If you can't stand the sight of the finger cot, you could substitute a small fuel filter, but it will not be a sealed system. Form an arch in the tubing, so the cot is lower than the top of the arch. Keep the arch as high as possable to prevent water intrusion in the event of submersion and cot failure. Secure the end of the tubing to conveniant point.
  6. (optional) Seal MC access cover with RTV. You won't need to use it anymore (thank God!).

"The system should still be flushed periodically to ensure longevity of the cylinders, and fluid level should be checked regularly. Also check the condition of the cot. If the cot gets fluid on it, it will likely deteriorate. I check mine every time I get gas and check oil."

Joe Van Slyke has an additional suggestion: "I've got a 1942 GPW. Instead of the finger cot, I used a $5 brake bleeding kit that had a flat plastic bottom, and used some JBWeld (epoxy steel) to mount a brass fitting into the bottom of it. And made a bracket to mount it on the firewall. Doesn't leak a drop. Let it sit for 2 days before I used it. I ran a 1/4" copper tube to a fitting on the MC cap, and just fill the small reservoir about half way. Works pretty good. You can see the reservoir on www.flatfenders.com. Look at the engine shot, lower right corner.

"But since then I found out a 1997 Ford F150 with a standard transmission has a small reservoir for the clutch slave cylinder. I looked at one on a used car lot here in Austin, TX -- it's a one piece molded unit with a mounting bracket built in to it. One of these days I'll track one down in a junkyard an put it in instead of what's in there. The reservoir has a short 4" hose on it, and runs into a tube to the slave cylinder on the tranny. Since it's designed for use with brake fluid I figure it's safe enough."

Ron Morris in Washington state adds: "I tapped the lid on the master cylinder and added a barbed fitting, then ran rubber hose up to the fire wall where I mounted a small plastic transparent reservoir used on VW busses for this same purpose (it's located under or behind the drivers seat). I installed this setup 15 years ago and have never had a problem at all. Easy to see the level, easy to fill, and extra fluid capacity doesn't hurt either."

Trouble with brake lights?

Brake Light SwitchJon Paulsen: "Having trouble with your plug in contacts on your master cylinder mounted brake light switch? I sure did. Every 6 months I had to unplug them and clean. Finally, a few weeks ago, the little clips wore out and I couldn't get a reliable contact at all. Time for a New Solution!

"The New Solution: I took a 10-32 tap, tapped the holes in the switch, soldered terminal lugs on the wires and bolted them on. It works great. I used longer than necessary bolts [about 1 in.], put 2 nuts on each bolt, with the terminal lug and a toothed lock washer between [the washer ensures a good contact with the nut/bolt]. Then I turned both nut [loosly] close to the head of the bolt, screwed the bolt into the switch until it bottomed, and finally tighted the nuts together to sandwitch the terminal lug/lock washer. Very secure. Just don't over-tighten the bolts into the switch, as it is copper and will strip easily and does not need to be very tight for good contact."

Andy had a problem after rewiring his CJ-3B. He stated: "I have rewired the '56 and the only thing that does not work is the brake lights. I might have something backwards, but can't figure it out. Everything is hooked up the way the instructions show. Can it be something in the master cylinder? Everything is new, even the switch on the end of the master cylinder."

Ed Wilson suggested: "If you have the original master cylinder, there is an hydraulic switch that screws into the master cylinder. Is this switch new too? If all else is new and correct this could be your problem."

Bob responded: "Try putting a voltmeter on the screws and checking for resistance. Have someone push the brake pedal for you. If the switch works, then the wiring is your culprit. Make sure one of the wires is hot coming from a power source, and the other goes to the lights.

"Also, if I am not mistaken, the brake lights run through the directional switch on the steering wheel. I had a problem and thought it was the master cylinder switch, but found the directional wiring in the box to be the problem."

Wes K had another option: "If you don't have a voltmeter just take a test lamp and ground one of its wires and then touch each terminal on the brake light switch with the other test lead while a helper steps on the brake pedal. Both should light the tester. If only one terminal lights the tester, then the switch is bad or stuck. If neither terminal lights the test lamp, then you need to check your power source. Note, to change the switch you must unscrew it from the master cylinder. The master cylinder will have to be bled when the new switch is installed."

Bruce W said: "Mine takes lots of foot pressure to light the brake lights. If this is the problem then the switch should be replaced."

Sam Nielsen replied: "Make sure you have a good contact between the brake light and the body, since this would be the light's ground. The input voltage goes through the brake switch at the master cylinder and applies positive voltage to the brake light when the pedal is pressed. However, even if that switch is working, the light needs to have a good source of ground."

Preventing Overheated Brake Fluid

Joel Kamunen observed on the Bulletin Board: "After driving my 3B up to operating temperatures, it appears as though the exhaust pipe is throwing off enough heat to expand the brake fluid in my master cylinder and makes the brakes (all four) drag. Has anybody else had this happen? Any ideas on a fix for this?"

George Romond answered, "I've never had this actually happen. But even if the fluid is expanding in the master cylinder. I think it should freely return to the master cylinder. Mabe the fill cap is not properly vented causing a presure to build. Also the master cylinder should have some room for air (not overfull). Possibly a combination of both is the cause."

Bob W. noted, "If the push rod to the master cylinder is adjusted too long, the fluid can not return to the reservoir because it is just like you are stepping on the brake. Make sure that when the pedal is released there is slight play between the push rod and the piston in the master cylinder."

Reed Cary asked, "I wonder if you are using silicone brake fluid. This seems to be an inherent problem with using it, from what I've read: expansion under heat."

Jyotin mentioned, "There is a shield that bolts to the master cylinder front bolt and to the steering box bottom bolt that shields the master cylinder from the exhaust. It is about 8"x 4" and almost rectangular except for a small piece that was sliced off at one corner to make it fit. Many times this shield is missing."

Where's the Leak?

Miles stopped by the Bulletin Board looking for some advice. "I have a 53 3B and I've had some brake problems lately; it barely stops. Everything is still original and it leaks some fluid by the master cylinder so I ordered a new one. I am wondering if anyone thinks it could be something other than a bad master cylinder."

Will recalled: "I had the same problem with my 2A. The master cylinder leaked, so I replaced it, but still had bad brakes. I looked in the manual and there was a section on adjustment. Mine has slits to adjust them and it stops real well now. I think if you adjust them and they're still bad, you should pull the drums and look for seals that are bad, letting grease get on the shoes."

Rus Curtis responded: "If you are getting pedal pressure and the jeep won't stop, then with or without a leak, the master cylinder is working. If you're not getting pressure (need to pump the pedal to get pressure and if held, the pedal bleeds to the floor or reservoir constantly needs refilling) then yes, possibly.

"You will need to pull the hubs to check the condition of the wheel cylinders and shoes. Is there a wheel cylinder leaking? Is there grease on the shoes from the axle gasket? Anything on the shoe lining and they won't work efficiently. Fix whatever's leaking on the shoes then replace or reline (do the entire axle when replacing brakes) the shoes.

"Now if your master cylinder is leaking but you can pump the pedal in order to get pressure, then a rebuild/replacement of the master cylinder and a full system bleed, to remove air bubbles, would fix the pedal. Adjust the shoes according to the manual and the brakes should work."

Note: steering and brakes on your vehicle are critical to life safety. The information on this page is general in nature; if you are uncertain where your problem lies or how to correct it, get professional advice.

Thanks to all the contributors. Illustrations are from the Willys Parts List and Service Manual. -- Derek Redmond

Also on CJ3B.info, see Dual Reservoir Master Cylinder Conversion.

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Last updated 3 May 2009 by Derek Redmond redmond@cj3b.info
All content not credited and previously copyright, is copyright Derek Redmond