Did a brake job on all 4 wheels and changed master cylinder, but still have a low brake pedal

Did you properly bench bleed the master cylinder itself before installing? It must be bled or there will be air trapped inside the master cylinder itself that no amount of wheel bleeding will remove.

Does the brake pedal go to the floor? Or does it stay up, and is hard to push...? Did you bleed the master cylinder? Also, you will need to bleed all 4 wheels connection to the brake wheel cylinders...you must start with the rear, farthest from the master cylinder, then other rear, then front farthest from the master cylinder, then other front...the bleeding of each line is a must, since you switched the master cylinder.

Sounds like you may have a hydraulic leak somewhere. Check the rear wheel cylinders and front brake hoses going to the calipers.

Check the brake fluid level in the master cylinder. If it's low or empty refill it and check for a leak. If the master cylinder is full of fluid the cylinder itself could be bad.

start from the fartherest place from brake master cylinder, that will be the rear passenger side wheel. it takes two people to bleed brakes. you will find the bleeder valve screws at the top of inside of the rear drums on both rear wheels, have someone pump the brakes up, will take at least 4 pumps of brake pedal, at 4th pump keep pressure on pedal while you break loose the bleeder valve on the rear wheel, fluid will come out and brake pedal will go down to floor, keep pressure on brake pedal until you've tightened the bleeder screw back [note] you'll only have to back the bleeder valve out about a half turn until you see the fluid come out. after you've done the rear brakes add brake fluid to the master cylinder as it will be low. then go to the front passeger side and you will find the bleeder screw on the brake caliper. repeat the same proceedure as back. remember that pressure on the pedal is a must until you've tightened those bleeder valve screws back or else it will **** air into the brake system. suggest that you bleed all 4 wheels not just the back. after you've bled the front make sure you add more brake fluid back to the master cylinder, it will be low again. hope this helps and thank you for choosing fixya.com

Most likely you need brake job on front and rear. to test if the pads are worn on the rear set the emergency brake then push down on the brake. does it come up? if it does This means the rear pads are bad and most likely the front. You will want to check the master cylinder for fluid if it is low you will have to replace with DOT3 Fluid once it is full you may have to bleed the system to see if the brakes come up. If the fluid is ok and The pads are not wore out then you will have to check the master cylinder to do this you need to jack up the car one wheel at a time and test by pressing brake pedal down slowly till the wheel will not turn. If the wheels stop on all four wheels when you have done this test then you will need to replace the pads and shoes. If the master cylinder is defective either the front wheels won't stop or the rear won't stop turning when you press the pedal down. The Master cylinder is $39.00 with exchange, but you want to make sure the pads are good the rear shoes are good and the brakes are adjusted. Here is the web page for the parts you need. If you need help doing a brake job let me know and I will give you whatever information you would need to do the job with. You can test the master cylinder if you have rubber gloves too, just pop the lid off the master cylinder and have someone slowly push the pedal down with your finger on the front port see it you get fluid blowing out the hole when the pedal is pushed down. If you do the cup is bypassing and it needs to be replaced. Hope this helps.
http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/subcategory_Brake-Hydraulics_-49997377_13557_10151_10051_-1___

adjust your rear shoes up so that the rear drums just turn, with them ajusted to a low setting the brake pedal will be low .
also when you start your engine it is noral for the pedal to drop because of the boster ! [ but not to the floor ]
Bruce Pike

This means that you either still have a leak, or there is still air in the line. Be sure that you bleed a lot for a master cylinder change out. Also if the bench bleed was not preformed then there will be a tremendous amount of air in the system. A bench bleed is simply filling the master cylinder and priming it before you install it. Hope this helps. Good luck.

Ok the BRAKE BOOSTER is ONLY there to give you pwer assisted, otherwise NO POWER BRAKES. ( not anything to do with the low brake pedal ) Hopefully the junkyard master cylinder was in better shape than your original. AS far a bleedinda system, heres what I do. First with a1998 vehicle, I would go to ALL 4 wheels , OPEN & CLOSE slightly each bleeder to insure none of them are FROZEN, and they do freeze shut ENSUE THE MASTER CYLINDER IS FULL and CLOSED Now with an assistant sitting in the drivers seat, ready to pump the brakes, and ABLE TO HEAR YOU. Go to the furthest wheel from the master which would be the R/R. DONT touch anything yet. Tell the asst too slowly pump the brake pedal till it gets hard, and then HOLD THE PEDAL DOWN ( and dont let up on it until the pedal drops. If the ast lets up before the bleeder is closed you **** in air.Repeat at each wheel til the pedal stays hard. Hope this helps

Any leakage at the brake wheel cylinders? If so, you must replace leaking wheel cylinders.

More likely is a worn master cylinder. Replacement is necessary. This is not typically a diffucult job, but important to do it right. It sounds like you have the brake bleeding down, and you will need to do this after the master cylinder replacement! Be sure to bleed all 4 corners!

Regards,

Brian