How To……..Replace Brake Master Cylinder/Brake Booster

Or, if you have a brake pedal going to the floor and NO external fluid leak, you need a master cylinder.

1. It IS possible to remove the m/c by itself, I have done it more than once. I did not remove the m/c reservoir (just one more part to drip brake fluid) or any other parts in the engine bay, I DID pack shop rags under the brake lines, knowing that there WILL be dripping fluid from the lines, and I had a small assortment of plastic caps that I was hoping I could quickly push onto the lines to stop the drip. Also make sure you have the fender covered, because, as you know, brake fluid will dissolve paint in a few minutes if left to sit.

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2. You will need to gently pull the 2 wire leads from the “low brake fluid” sensor on the reservoir, then use an open 11mm wrench or flare wrench to break loose the brake lines before you loosen the m/c. I did not have a metric flare wrench so I used a 7/16 flare wrench, worked fine, crack them loose now, but snug them so they don’t drip. Use a short 13mm socket, first a 3/8 drive to break the mounting nuts loose, and then run them off with a ¼ drive setup, either way you will also need an extension and a flex joint. Have a “magnet on a stick” handy when the m/c nuts come off, don’t let them fall, there is also a lock washer behind each nut to catch.

3. I did not need to remove the vacuum switch and pipefitting going into the booster, but if you need more room, loosen the large hex nut right at the inlet pipe with a 7/8 wrench and it comes out with the switch still attached.

4. So, the mounting nuts are off, loosen/remove the brake lines, cap them off if you can, tuck them under the m/c but don’t bend them any more than you must and pull m/c slightly upwards as you pull up and out to remove. Carefully remove the wet rags and trash them, while not allowing one single drop of fluid to hit the car, use a can of brake cleaner and spray the inner fender completely and let the cleaner drip onto a towel or metal tray under the car, then dry with more clean shop rags.

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5. THIS is the problem with taking the m/c out alone……there is a long tube extension on the back of the m/c, as you gently pull the m/c out there is also a long push rod from the booster that may be stuck into the end of the m/c, or least will try to move forward with the m/c as you pull, I used a really long needle nose plier to hold that rod still, or even push it back into the booster. (Don’t worry, it won’t drop out) A wood or plastic popsicle stick, or something like that would work also….as soon as you can see some grooves or the adjusting tip, you will be free. It’s just the way they designed it, the two parts fit together fine but remember you are trying to “flex” those two separate pieces of steel as they slide out of one another, they will bind, that’s the problem, if you had 1-2 inches more of straight forward movement, there would be no issue.

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6. After my m/c was disassembled, there was pitting in the rear section of the cylinder bore, probably from water sitting there for 34 years, the m/c sits slightly down at the rear when mounted, any moisture sits at the back and causes rust inside. So, a new m/c and reservoir were ordered from jolly old England, and the wait begins. I had already paid for a spot at the FCA Abacoa Car Show on Nov 3rd, and of course the parts show up the day before the show. Not wanting to rush the repair and screw something up, I drove the Cherokee to the show, met some nice people, saw a bunch of great cars, and visited a plot of grass that should have had my car parked on it…..oh well.

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7. Here is the new m/c with the new reservoir attached. Notice the low level in the rear fluid compartment…..and the nice screw type plugs in the brake line ports. There is a divider wall between the front & rear compartments, you will need to “force feed” fluid into the rear half, I used a medium syringe I had left over from giving the cat a liquid medication, (it is lying on the white rag under the front of the m/c), suck new brake fluid up about half way and while the m/c is in the vise rolled over on its side, inject the plunger into the rear port and slightly push the m/c push rod in & out, you should see fluid disappear and begin to fill into the rear compartment. There is no cutaway or port (that I could find) to connect the two compartments, however, the rear half is “fed” from the front half, but you need fluid in the rear of the m/c to start the pumping action to MOVE fluid to the rear compartment first!

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8. I also had the local brake shop fab two bleeder lines which I can tighten into the brake ports with the open end sitting inside the reservoir, fill the reservoir with new fluid and press the m/c push rod all the way multiple times until there are no more bubbles coming out of the pipes, that means no more air in the m/c and more fluid in the rear half of reservoir.

9. I loosened, but left those nifty screw plugs in, then remounted the m/c, (I did spray a bit of white lithium grease into the m/c push tube), leave the m/c mounting nuts loose until you get the lines back on. That will let you have some “wiggle” room to get them started. And pack rags under the lines again and be ready to spray everything down with brake cleaner when you are done. Unscrew the temporary plugs one at a time and install the brake lines. I cannot stress how important it is to CAREFULLY start the line nuts by hand ONLY ! These threads are very fine and can easily be cross-threaded, but, since you did not bend the lines on removal, they should start easy, but some movement may be required, after the lines are on and snug, tighten the m/c mounting nuts.

10. Get your helper/wife/gf/somebody to do a final bleeding, engine off, be ready to crack one line open and tell them to start slowly pushing down on the brake pedal, open the line nut but before they reach the bottom of the stroke, you need to tighten the nut, they can push a couple of times to get the feel of the pedal, and do it again…keeping the m/c full of new fluid until you don’t see/hear any air or bubbles coming out of the fitting, repeat for the other line. Then start the engine and check for a solid pedal, and that there is no brake warning lite on the dash.

And of course, Part Numbers…..

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***** Now, what if you install a new master cylinder and then find when you start the engine, you have a very hard pedal AND you hear a sucking vacuum sound from under the dash, then you also have a bad brake booster. The booster is exactly that, as you push the brake pedal down, engine vacuum is applied to the forward side of a big rubber diaphragm held between two metal housings, with a vacuum on the forward side, atmospheric pressure presses on the back of the diaphragm and “helps” your foot press harder.
It is almost impossible to use a disk brake system without having a booster of some type, either a vacuum or hydraulic type, the required effort on the pedal to apply the force needed to stop is beyond what a normal human can apply.

So, with the m/cylinder already off, slide the drivers seat all the way back and crawl under the dash, you will be able to see 3 of the 4 nuts holding the booster to the firewall. Also look at the brake pedal linkage, as you push the pedal with your hand you will see a round rod/pin with a wire cotter pin bent thru a hole, twist/bend/pull the wire out and remove the pin and a washer which connects the booster push rod to the brake pedal. Now refer to a prior post on “How to” get the upper left booster nut off and…..lift the booster up out of the engine compartment and send it to be rebuilt.

To reassemble everything, just work backwards, however you can slide the m/c onto the mounting studs on the booster before you lower the whole assembly into the engine compartment, hand tighten the m/c nuts just enough to hold, but loose enough so you can wriggle the m/c a bit while you thread the brake lines back on. When you need to put that last difficult booster nut back on, use either a magnetic socket or a piece of scotch tape to hold the nut and washer onto the socket.

Questions ?

Gary