MGB Sponge Brakes

During the various test drives as I solved for the various leaks I described in my last post, the brakes have been spongy. If I lightly pumped the brakes a couple of times before using them, I could stop well, but that's not a safe situation. Before I could go on a real test drive, I'd need to fix them for real. Today's post covers that.

Master Cylinder pre-fill
The brakes were spongy since I completely re-did them. after bleeding and re-bleeding the system at the wheels, I concluded that there was air in the master cylinder. I hadn't bench-bled the master before install, so it seemed pretty likely that the issue was there. In retrospect, I should have pre-loaded the master cylinder with brake fluid by the process folks call a "bench bleed". This is where the master cylinder is set in a vice with the shipping plugs still in it. Using a pair of pliers, the activation lever is moved back and forth while brake fluid is added through the reservoir. Once the chambers are filled, the master cylinder is installed into the car. In the MGB, the master cylinder fits onto the end of the brake booster, so this could have been done.

Getting the hard lines attached can be an interesting challenge. As I look back on my efforts to get my master cylinder in, I suspect I would have leaked brake fluid all over the engine compartment. So, while I probably could have done it the "right" way, I think the way I solved it could actually have saved the paint on my driver inner fender.

de-Sponge
So, since I didn't do the bench bleed, nor think much about air in the system at all other than pulling it out at the wheels, I figured it had to be air in the master cylinder. To solve, I took the bleeder hose from the MityVac kit. It's about 3 feet (1 meter) long. I put a coffee filter on one end to catch particles from the hard lines and dipped it into the master cylinder reservoir. I put the other end on the driver-side front wheel bleeder. Then, I pumped the brakes moving brake from the reservoir through the master cylinder, down the hard line, then the soft line to the brake caliper. From there, it passed out the bleeder into the clear hose and traveled back up into the reservoir. The key here is to keep the coffee-filter end in the fluid in the reservoir. Without that, air stays in the system. So, keep topping off the reservoir and keep pumping the brakes until there wasn't any air in the clear hose.

This worked great, except I couldn't close the bleeder without re-introducing air. I tried 3 times before I had spread brake fluid all over the inside of the tire and the garage floor. I succumbed to the obvious: remove the tire first. Yes, I failed to both mention and do that above. At any point before you connect the clear line to the bleeder screw you should remove the driver side front wheel. Why yes, that does sound obvious. If you don't, you'll be like me and try to do this with limited success because getting the bleeder closed without letting in air when the wheel is still on is virtually impossible.... especially if your car is on the ground (not on a lift), and its on the ground in a garage that's full of projects, parts and tools... and sports equipment... and camping gear... and laundry machines.

Anyway, after washing off the brake fluid, I set up again, and had the line bubble-free on my fourth attempt. This time, I was able to get the bleeder closed. I proceeded to quick-bleed the other 3 corners, but virtually no air bubbled appeared. Another test drive, and the brakes were nice and responsive. I haven't tried the stomp-on-the-brakes test that the car shows on Velocity TV do, but I will be next time I take the little car out.

That's it for this time. Thanks, as always for following along--