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Q: I own a nice '74 MGB and I am having trouble with the rear brakes pulsating when applied especially with force. Please give me any info you can so I can remedy the problem. Thanks.
A: Hydraulic issues usually involve air getting into the system or fluid leakage somewhere. If your shoes and pads are in good shape and you still have to add brake fluid fairly regularly (like weekly), you might still want to check for leaks and bleed the system first just to see if any air comes out and it improves. But if your brake fluid level has been constant (adding fluid only for normal wear) and no leaks are evident, I would think a this more a mechanical issue.
Pulsating is more often to do with the drums being out of round or the rotors (front brakes) being warped or having a high spot that catches and then releases. It could also be a difference in the surface, such as damage from rust after sitting too long that creates a drag and, at the smoother portion, a release.
Rotor warping and drums being out-of-round are more common on older cars because the rotors have been cut, possibly more than once in their life, so are thinner and more susceptible to heat damage from hard braking and even normal use. Many shops working on newer cars these days don't even turn the rotors for this reason. They just replace them outright.
It is hard to tell while driving if it is the front or rear that is the problem. I would suggest you start by removing the drums and rotors and having them checked for trueness and thickness and, if within tolerance, then turned to be true. If the rotors are warped or have a hard spot (again only to be found if checked and turned), they will have to be replaced.
Plan to replace at least the shoes and pads at the same time you cut or replace the rotors and drums. Be sure you also check the calipers, wheel cylinders and axle bearings for leakage, rust, seal damage or binding. A bad axle bearing can also cause a pulse, but is usually noisy by that point.
If it has been a while since any work has been done, I would think the pad/shoe, rotors/drums would be the more likely candidates for the source of the problem. I would add you consider rebuilding the front calipers and replacing the rear axle wheel cylinders while doing this work as a precaution if their condition is at all suspect.
I hope that helps you. Keep me posted and let me know what you find.