help with brakes

[bingslayer]

Hi, Ive been having some trouble sorting out the brakes of my 90 Bighorn.

My brakes seem to have lost a lot of stopping power, with hardly any power if I press more than once in a short space of time.

The check light also flicks on when braking hard, and will stay solid on the 2nd time braking (as above).

I have tried another booster, bled the lines,visually checked master cylinder,lines and calipers for leaks and checked for vacuum.

there is also no fluid loss in the master cylinder, but im assuming that I would have low pedal pressure problems if there was a leak? The pressure seems fine.

Anyone have any suggestions?

[geeves]

does the peddle go just about to the floor?
If so check the slides on all the calipers. One is the bolt the other a sleeve the bolt goes through. If these are siezing you end up with excess travel in the calliper.
If no I would be looking for a vaccum leak or a bad vac pump on the alternator

[bingslayer]

The pedal pressure seems pretty good, but there is little braking power.

regarding the vac, all I have done is disconnect the hose from the booster and felt a good (how good is good?) amount of suction on my thumb. The hose doesnt seem to have any leaks either. where should I be looking next?

I also too a quick drive with hose disconnected, there is a significant difference in braking power so I think the booster is doing its job.

with regards to the pumping of the pedal and losing most braking (pedal going hard) on the second push, does that sound typically vac related or is there a possibility of there being a fluid,caliper or master cylinder problem?

[geeves]

There is a non return valve in the booster If it fails you might have problems for the first minute or so after starting the engine. Easy fix is a non return valve from any petrol car in the line. This also masks minor vaccumm leaks. What happens if you do the vac test again but turn the engine off 10 seconds before removing your thumb. If the vac has gone you have a leak.

My most likely thought though is that you have unevenly worn pads or if you have just changed pads they are either poor quality or havnt bedded in yet..
Things to check
Visual look at all discs. Shiney is good because it means you have pad contact. A slight roughing with fine emery paper wont hurt
Look at all pads. Looking for thickness but more importantly are they wearing evenly. If they are tapered this would indicate a problem with the caliper slides. Depending on how bad they are you may have to replace pads after fixing the real problem.
Check caliper slides The caliper slides on one bolt but the other bolt goes through a tube which must slide in the caliper. I have to clean free and relube these on my bighorn every 2 years despite putting new seals in.
Check the pistons. A siezed piston will cause this. Fronts try and push back. It will take reasonable force but if you cant push back its siezed. You might be able to free it with brake cleaner but a rebuild is a better answer. Rears depend on which rear brakes you have