Introduction #
Brake Booster Replacement may be necessary on the USA specification 1977-1980 MGBs (so-called Mark IV) which had this equipment.
I had noticed an air noise/hiss each time I pressed on the brake pedal. My suspicion was the brake booster. As far as I know, it is still the original one since it has the factory sticker on it about cleaning before doing anything with the brake fluid.
Some research and some tests to try and verify it was indeed the booster were performed and made no change. So, I started looking for replacement options. After some research, I decided to purchase a complete unit from the UK supplier The MGB Hive. The main reason was that the UK was the only source of parts that would have the gold coloring coating to look like original. As I was to be visiting I could collect it in the UK.
Step 1 Prepare & Remove the Brake Master Cylinder #
I have previously replaced the brake master and followed some of those steps again. Mainly I used some aluminum (tin) kitchen foil to run down the inside of the fender and protect the paint from brake fluid. I inserted this behind the brake lines and ran it from the top down to the frame rail. On the bottom edge and sides, I created a channel to catch fluid rather than have it get onto and remain on the frame rail. I also removed the air filter assembly to create room to work. Also I extracted the booster hose from the manifold. Some protection for the fender (wing) is also required. Because of working with brake fluid I’d use something nonabsorbent e.g. – not just a towel!
Loosen the 2 nuts that attach the brake master to the booster and then remove the brake hard lines from the master cylinder. Make sure to get all 3 of the hard lines broken loose to start. Don’t get one fully out and then find you have to wrestle to remove one stubborn one. The brake master will drip fluid and you don’t want it to run dry while you fight with a non-cooperative hard line.
If you have some appropriate plugs, use these to plug the master cylinder ports as you remove each hard line. I also capped each line with a selection of vacuum caps to keep out any dirt and stop drips. Then remove the 2 nuts and remove the brake master completely.
Step 2 Remove the Booster Itself #
This is where stuff gets a bit interesting. The booster is attached to the pedal box with 4 nuts in a square pattern. Two of these are fairly easy to reach from the top of the pedal box (having removed the cover and TCSA switch [if present]). The other 2 are more difficult. I consulted with John Mangles (The MG Doc) on approaches. I was hoping to get at the two bottom nuts without having to remove anything else using a wobble adapter and socket and maybe the same from inside the car. John suggested removing the pedal box and disconnecting the clutch master at the banjo bolt. I don’t usually ignore “the Doctor” but decided I’d try my contortion approach first. Luckily I was able to get it done.
I was able to get the bottom passenger side nut loose with a wobbly and socket/extension and finished with a wrench (spanner). The same wobbly and socket from inside the car, through the access area for the pedals, to the bottom driver’s side nut also worked. I used a magnetic pick up to help fish out washers and nuts.
The last part of removal is the connection from the booster to the brake pedal itself. For this there is a cotter (split) pin that needs to be removed and some long needle nose pliers did the job there to close the cotter (split) pin and remove it. The pin conveniently fell through onto my rubber drivers floor mat. The washer was retrieved with a magnetic pick up and the clevis pin pulled out from the connection with the long needle nose pliers again. Some pictures of the pedal box below may help understanding this process.
Step 3 Transfer of Bits #
From a Tech Session with John Twist, I know that there should be a rubber seal between the brake master and the booster. This is to prevent a vacuum leak to the engine. He kindly sent me one as they were N/A at the time. Moss Motors, currently, seem to have them (180-789). I removed this from my old unit, and you may also need to extract the metal casing for it. Then transfer it over along with the hose from the one-way valve (that goes to the manifold).
The new booster came with 6 nuts for the mounting of the booster to the pedal box and the brake master to the booster. I’m always leery when this happens as it can often mean an unusual size or worse, metric! Yep, although not measured 12mm seemed to be a better fit than the ½ inch sockets I had been using. The thread on the mounting posts was also different and the nuts had an integrated lock washer. Prepared with some metric tools I could proceed.
Step 4 Reinstall is Just the Reverse of the Removal Process – That Old Gem #
I started with letting the booster sit in place and tackled the connection to the brake pedal with the clevis pin first. Tried several times to put it back the same orientation as it came out. There was just not enough room on the right/driver’s side to keep the clevis pin horizontal, given the pedal box construction. There was more room for this with the needle nose pliers from the other side and it popped straight in. I used the magnetic pickup to position the washer and then the needle nose pliers and a screwdriver to put back and secure the cotter (split) pin.
Next up was the 4 nuts to secure the booster. Since I didn’t have a 12mm wrench (spanner) I had to use a socket. I got all 4 nuts in place by hand first from above for the top 2. From inside the car for the bottom 2. I then used the socket and wobbly with a variety of length extensions and got them all secured.
Next up was the brake master.
Tip. You may be tempted to mount the brake master to the booster and then re-attach the brake lines – DON’T.
There is actually a fair amount of movement or play on the hard lines (strange perhaps but true). You must be really careful not to cross-thread the hard lines to the master. Also the angles needed to reattach and get things lined up are difficult. It’s better to work on getting at least the 2 close together hard lines started while holding the brake master in your hand. This lets to move/adjust the angle of the master to the hard lines. Once they are started enough to not come out you can loosely position the brake master onto the booster.
Next take care of the third hard line connection (closest to the front of the car). Being able to maneuver the brake master will help get the hard line square and started correctly. Once they are all started, you can bolt the brake master firmly but finger tight to the booster. Then tighten all the hard lines up fully snug. Assuming no drips of brake fluid, snug down the 2 nuts holding the brake master to the booster with a socket or wrench (spanner).
To complete I just had the brake booster hose to connect back up and put the air filter back in place, not forgetting the return spring to reattach to the carburetor.
End result as is I had hoped. No more air hissing noise and no vacuum leak when pressing the brake pedal. Although, I now don’t have the brake fluid warning sticker, at least the booster is the right gold color and I may think about opening the original and seeing if a repair might be possible. For now Brake Booster Replacement is complete.
Other Materials #
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