Over the past year I’ve dealt with a Spectra that had multiple brake failure issues. All of the hydraulic parts except for lines and proportioning valve have failed and once I even found the parking brake cable to be severely overtightened. I’ve seen many brake problems with this car and can see which are likely to cause your symptoms. This is what I think you should do:
First, I am going to assume you haven’t been seeing an excessive fluid level drop in the brake fluid reservoir. If you have, then you’ve got an easy to find leak.
Note that where I say to replace a component which is rebuildable, rebuilding is fine. After any replacement or rebuild of component, start this check again at the top. Let’s fix this thing before anybody gets hurt.
-Verify that no wiring (such as for after-market equipment) or floor mat limits upward travel of the brake pedal. Correct any problem found.
-Bleed the brakes. I understand that you’ve already done this but do it again. Do not use a pressure or vacuum bleeder, do it the old fashioned way. Open valve, pedal down, close valve, pedal up. No pumping. Always bleed with the reservoir cap loosened 1/4 to 1/2 turn. If you cannot get five consecutive squirts with good strong flow and no air at any one wheel, replace the caliper or wheel cylinder at that wheel and bleed again. If you cannot get five consecutive squirts with good strong flow and no air at two or four wheels (not one or three) replace the master cylinder, reservoir, and the rubber grommets where they meet, then bleed again.
-Wait 30 minutes, then bleed the same way again. If any air at all is present at one wheel replace the caliper or wheel cylinder at that wheel and bleed again.
-For each caliper:
Remove the caliper without disconnecting the hose. Use plastic (not metal!) hose clamping pliers to pinch off the brake hose. Using a suitable tool (be creative if you have to) grasp the caliper piston and try to pull it out from its bore. Pull for a minimum of 30 seconds. Put the caliper back into place, remove the hose clamping pliers, and bleed at that wheel. If any air is present, replace the caliper and bleed again.
-For each wheel cylinder, check for any wetness or corroded metal behind the dust boot. Check that the pistons move freely in and out (one moves in as the other moves out) with no roughness or binding. If any wetness, corrosion, roughness, or binding is found, replace the cylinder and bleed the brakes.
-Remove the brake fluid reservoir cap. Check that the rubber diaphragm is properly seated on the reservoir fill opening, is not swollen or oily feeling, and has no punctures or tears. Replace it if otherwise. If oily feeling, first thoroughly flush the brake fluid out with new fluid, then replace all calipers, wheel cylinders, and hoses, as well as the master cylinder, then bleed thoroughly.
-Note the hole on the outside of the reservoir cap. Verify that you can easily blow air through the hole. Insert a small tool into the hole from the outside, and verify that you can see the tool from the bottom side of the cap. If you cannot blow through the hole or cannot see your tool, repair or replace the cap as needed then bleed the brakes. I mean that. Bleeding the brakes is necessary if the vent hole in the reservoir cap has been found to be clogged.
-Check the length of the adjustable push-rod between the master cylinder and the vacuum assist booster. Verify that it does not turn (adjust) too easily. If it adjusts too easily, deform the threads slightly so that it does not, then adjust correctly. If it is too long, first adjust correctly, then bleed the brakes.
If you skipped part of this because it didn’t make sense or looks like a waste of time, start over at the beginning. My wording may not be the best, but there are NO extra steps laid out in this plan. All of it is necessary. Do not assume that finding and correcting one problem will fix it. When dealing with a problem car, just when you think it is fixed is the time you need to start checking everything from the beginning.
Hopefully the problem is fixed at this time. It would be good to use the car for a bit and make sure there is no sign of any problem recurring before returning it to its owner.