Have tried everything....have 1990 Nissan Stanza. Have replaced master cylinder which I bench bled prior to install. All lines were bled. Still can't get pedal to stay hard. Any advice would be appreciated, what to try next.
"In less enlightened times, the best way to impress women was to own a hot car. But women wised up and realized it was better to buy their own hot cars so they wouldn't have to ride around with jerks."
If you're losing any brake fluid you may have a leaking wheel cylinder. When you were bleeding the brake lines were you keeping the fluid reservoir topped off? If not it could have gotten low enough that it sucked more air into the system. You should refill the reservoir after bleeding each wheel to be sure it stays at a level full enough not to pull more air in. There could also be a leaking brake line either rusted through or leaking at a connection.
When replacing the master cylinder, you have to bleed all 4 wheels. Its also a good time to flush the system completely. I did it myself by placing a small glass with some fluid under the wheel and running a hose from the bleeder to the glass making sure it was submerged in the fluid. Open the bleeder at that wheel and pump the master 3 times. Check the fluid level in the master and keep it topped up. This blows the crud out of that wheels circuit. Do it twice for each wheel.
If that doesn't cure your soft pedal then you may have rear brake shoes that are not adjusted properly. If the adjusters are working properly you can back the car up or pull the emergency brake to adjust them. If not you need to adjust them manually. They tend to get stuck on a car that old.
In the front calipers the pins that allow the calipers to float may be frozen. The front caliper is a single piston, with the caliper held in place by two pins that allow it to move as the pads wear.
This is important. You can check each wheel to see if it is causing your problem, as long as you are sure there is no air in the lines. Clamp the brake flex hose with vise grips, preferrably those with smooth jaws. Dont get too vicious with the clamping of you could damage the hoses. Clamp each wheel individually and then press the brake pedal. If it becomes firm, then that wheel is the cause of your problem. Sometimes it is just one wheel other times its more than one. We would clamp them all off, then the pedal would be hard as a rock with almost no travel. Releasing each wheel and retesting would tell us which wheel was the culprit.
If you have doen everything else and still can't get the pedal firm, and you KNOW there is no problem with air in the lines or any out of adjustment rear wheel or frozen caliper pins on the front wheels. Then there is one additional step.
The Last step after all other problems have been investigated and corrected, is to adjust the length of the pedal to master cylinder operating rod. Longer for higher application and shorter for lower application. This is almost never necessary with OEM replacement parts, but arises with aftermarket parts.
I have done a few hundred brake jobs on 90 model Sentras and this procedure never failed me. Nissan brakes hoses almost never fail.
Gravity bleeding is fine. but it doesn't break loose the gunk that tends to accumulate because most people don't flush the brake system every two years.
It also wont work with some systems because the brake lines in some cars have upwards curves and normal bleeding will not blast the air bubbles out of the high portions of the lines like my method.
The mid 90's 300 zx twin turbo clutches had their own booster nad the lines had fairly large vertical rises in their pathway around the engine compartment.
After a Nissan master tech spent 2 hours trying to get the air out I showed him the recommended method and did it in 5 nimutes. When I converted my 76 Z to rear disc brakes my method was the only way to get all the air out.
One more helpful suggestion. When you are bleeding brakes but you have not replaced the master cylinder, and it is old, never press the pedal further than you would in normal brake application. Pushing it all the way to the floor can wipe out the seals and finish the old master off. Seen it happen many times.