MGB - Brake Booster Replacement

In my last post about Oliver, the 1978 MGB, I mentioned that the brake booster was the cause of a vacuum leak in the engine. This vacuum leak was causing cylinder #1 to run very lean compared to the others. Fortunately, Oliver has not seen much road time, so the engine should be fine. Regardless, today, I cover the fun of removing and then installing a replacement brake booster.

I would like to note that the body work on Zed came to a full stop when the Oregon weather predictably shifted back to cold-wet. For those who don't live here, we experience a wonderful donut-hole of beautiful weather in April/May smack in the middle of the cold-wet. The cold-wet returns for the month of June, usually lasting until Independence Day. So, I entertained myself with other projects, like this, while we waited for the weather to change back. After yesterday's high temps, I suspect the cold-wet is over until October.

Booster in an MGB

We start with where the booster lives in the small engine compartment of the MGB. The brake master cylinder, the clutch master cylinder and the brake booster all live together, attached to one another through the "pedal box": a rectangular steel box just in front of the firewall on the driver side. Closest to the rear is the clutch master cylinder. It is bolted to the back of the box with a lever extending into the box and attaching to the clutch pedal with a pin that is held in with a cotter pin. Attached to the front of the pedal box is the brake booster, with 4 1/2" nuts. This has a lever passing rearward, attaching to the brake pedal with a similar pin-with-a-pin design. To the front of the booster, the master cylinder is attached (2 nuts either 7/16" or 1/2"). Into the master cylinder a rod is passed from the brake booster.

You can imagine the brake action then: pedal is depressed which leverages against the clevis pin to press the lever into the brake booster. The brake booster converts some engine vacuum into more braking force which is then applied through the rod to the master cylinder.

Booster OUT OF an MGB

booster out

The Bentley manual has a step-by-step instruction for removing the brake booster, but I found the instructions to fall down. It started with removing the cover on the pedal box. I agree with that. It then described removing the clips for the brake lines leading to the master cylinder within the engine compartment. I agree with that. Then, it instructed to remove the bolts holding the brake master cylinder to the front of the booster. I also agree with that. At this point, the brake master cylinder can be slid forward off the booster-rod. By doing it this way, no air is introduced into the brake system. I likey.

Then, the instructions said to remove the cotter pin holding the clevis pin for the brake pedal to the leaver on the brake booster. This is not possible for anyone with adult male (and probably female) fingers for width and no one with child fingers for how deep you need to go. The gap between the side of the box and the partition which runs front-to-rear is not wide enough for fingers. I tried long needle nose pliers, but I concluded that even if I got the cotter pin free, and got the clevis pin off, I would not be able to re-install this way. So, I started my own path from here.

My plan was to remove the pedal box with the brake booster still attached and remove the booster from the pedal on my bench. Unlike the brake cotter pin, the partition is much wider on the clutch side, so my average-thickness fingers were able to remove both the cotter and clevis pins without too much difficulty. So, with the pedal disconnected, we remove the clutch master cylinder from the box by removing the 2 bolt/nut combinations that attach it from the rear.  Once removed, the clutch master  cylinder can be pushed rearwards out of the way. Next, we focus on the pedal pox. There are 2 bolts from the top, along the outer edge of the box nearest the centerline of the car. There are 3 bolts from inside the driver footwell going up. Last, there are 2 bolts from behind the dash running forward. Once all 7 bolts are out, the box is only held in place by gravity and the seal. Some light upward pressure on the brake booster and the unit pops free. Getting the unit out of the car took some wrestling, but I think this was more difficult because I could not see where the pedals were dangling. I found that by rotating the unit 90* clockwise helped the pedals to make it through the hole. had I read my old post about removing the pedal box (See MGB - Master Cylinders (Part 1)) or the post about the install afterwards (See MGB - Master Cylinders (Part 3)), I would have known to loosen the bolt fastening the clutch pedal so it dangled a little bit more. This would have made the removal much less of a muscle effort.

pedal box upside down on
workbench with old booster

Once the pedal box was out, it was a simple matter to remove the cotter pin and clevis pin between the brake pedal and the brake booster.... from the underside. I removed the remaining nuts on the brake booster, and started the install by performing the steps in the opposite order: attach the brake booster to the front of the pedal box. Thread the clevis pin through the lever and pedal, minding to include the washer before re-inserting the cotter pin.

Booster Back In MGB

With the unit in the same relative state of assembly as it was in when it was removed, I returned to Oliver to install it. I approached the install in the same manner as it was removed: rotated 90* so the booster was pointing across the engine. The pedals didn't fit, but with some rotating of the box and squeezing the pedals together, I could get them to fall through. Then, I wiggled and rotated the box, checking for cables wires and tubes from getting underneath until it settled in place. I decided that I would do the hardest first, so I did the clutch pedal/master first. I bolted the clutch master cylinder to the rear, after wiggling it around to fit, setting the lever alongside the clutch pedal. Then, I attached the pedal with the clevis pin and cotter pin. I found there was more play that I expected at this joint point, and concluded that I got the clevis pins backwards between the brake and clutch. Food for thought if you are doing this: the longer clevis pin is for the brake.

new booster in

With the clutch master cylinder tied in, I added the brake master cylinder to the front. This is much easier to get to. Once fitted, I started fitting the pedal box to the body. Because of the brake and clutch hard-lines, the pedal box is reluctant to move. I found that putting the 2 bolts through from the dash forward into the rear-most part of the pedal box allowed me to move the box into position. Once the bolt holes along the inside edge (2 of them, one 1/2" one 7/16") aligned, I set those 2 through. Last, I got back on my back inside the driver footwell and sent the last 3 7/16" bolts through the outer edge of the pedal box.

Brake Booster Hose

With the new booster on the pedal box and the pedal box in the car, all that remained was connecting the vacuum hose. I had oriented the booster upside down from the original so the vacuum nipple is on top, rather than below. I did this because of the significant heat source below the booster (exhaust) that is less present above. Also, the hose is much shorter and straighter from intake manifold bung to brake booster nipple. Wanting to avoid any possible issues with the original one-way valve on the intake manifold, I replaced it as well.

Re-Tune and Jetting

Because the old booster was creating a vacuum problem, I re-did the ColorTune tuning steps. Similar to the first time, I tuned using cylinders 1 and 3 but achieved a nice blue in both banks this time. I did swap out the idle jets (50F9 jets), but in order to find blue, I had the idle mixture screws further out than 2 full turns. I will drive Oliver around a little bit, but I think I may need to go up to 55F9 jets if the instructions in the SK Racing manual are to be followed. I noticed that the SK Racing product page describes the "pilot jet" in the carb set-ups they are selling today as 55. Assuming this is the idle fuel size, that's more support for trying 55F9 idle jets.

Since these SK Racing carbs are so similar to the side-draft Weber, here's another tuning link I found specifically for Weber. Notice how that article indicates the starting spot for the idle mixture screw is 2.5 turns out? That is about where mine are now. Curious. One last link, again on the much more plentiful Weber DCOE, but this time about some recommended jet sizes. It recommends the 50F9 idle jets for a late B like I just put in, so honestly, at this point, it will come down to a road test.

under the jet cover plate

Swapping out jets in this carb is seriously one of the easiest things I think I have ever done on a car. There is a cover plate on the top of the carb which is held in place with a slotted bolt. Once the cover is removed, the 4 jets (one idle per side and one main per side) are held in with slotted jet holders. The idle jets are the ones further from the engine / closer to the air filter. The holder threads out, and the jet is held in the holder basically with friction. The old jet slides out, the new jet pops in and you thread everything back together again.

The engine tune did not meaningfully change after the re-jet, so I concluded that the air-fuel mixture from the old jet was virtually the same. So either I removed a pair of 50F9 jets only to put in new ones or the old ones were a similar enough combination that they provided the same air-fuel at idle. At least I know what they are now.

All that is left is a drive... but wait! I didn't hook up the brake light switch. Well, I did, and after a few days of head-scratching trying to get the lights to illuminate, I realized that I had plugged it in wrong. The source 12V arrives in a dual plug, and the signal to the brakes departs through a single plug. I had thought the single plug was from the old carb-preheater, so I had failed to send either side of the switch to the brakes. Instead, I simply wired both sides of the switch to the 12V supply. Hahaha. Simple fix. Once the cars were shuffled so Oliver could get out past the herd, I did a longer test-loop. It did not take long to get the engine to normal operating temperature and he ran great. There was a slight stumble (bog) from dead stop, indicating that he was still a little lean. So, once back in the garage, I turned the idle screws about 1/8 a turn more rich. When I rev'd him from idle, I could not note a bog so I figure he is fairly close to tuned now. More road testing will confirm, and I expect I will fiddle with these screws as weather and altitude factor.

Thanks, as always, for following along-