My guess is that you either have not fully bled the master or you have a defective replacement.
How are you bleeding the master?
As a suggestion, the way I do it is to put the master in a vise on the bench, (be careful not to over tighten the vise if you are clamping on the master body.)
Run a line from each port to its respective reseviour. (I use a hard line as I know this will not leak and they will not pop out or flop around.)
If there is only one reseviour then run a line from each port to the single res
Fill each res with brake fluid so it is over the bleeder lines so they they can only suck in fluid and no air.
Push the pitson in slowly and release slowly, continue to do this until no air bubbles are seen coming out of the bleeder lines.
Sometimes stepping away for a minute or two to let the bubbles float to the surface will help. If you push the piston in and out too fast, you just circulate the bubbles around and around.
Once you know you have all air out. mount the master on the vehicle leaving the bleed lines in place.
Then, remove one bleed line at a time and quickly install the brake line. Make sure you have a rag or something to catch the fluid that will drip as you swap the brake lines.
Next, I gravity bleed the system. (You do this with engine off.) Fill the master with fluid to top line. Leave master res cap off.
You need a flexible hose that you can attach to the bleeder on the wheel cyl (one at a time). This needs to be a clear hose like the ones that come with a mighty vac tool, these are perfect for this and are ahout 18" long.
Attach one end of hose to the bleeder on the wheel cyl and then raise the loose end is higher then the bleeder valve. I hold it straight up allowing the hose to bend up just past the bleeder valve. You can use a piece of wire, or something similar to keep it raised but I generally just hold it in hy hand. I also have a container handy as well to catch the fluid once I'm ready to lower the hose.
The loose end of the hose needs to be open.
Now open the bleeder on the wheel cylinder. (sometimes you will need to install the wrench on the bleeder and then attach the hose.) You may need to open the bleeder valve 1/2 to a full turn. The fluid or air should slowly start to come out the bleeder and into the hose. With the hose raised, you can easily see fluid or air coming out.
Once you have just fluid and no air, close the bleeder valve, then lower the hose to drain it into a container, then move on the the next bleeder.
This will usually get 100% of the air out of the system. Don't forget to check and refill the master as you move from wheel cyl to wheel cyl. I use this gravity method because I'm generally doing it by myself and don't have a helper to work the brake pedal. But I found that using this method first before trying to use a the helper method a;most always gets out all the air and also allows me to see when the fluid is running out clean and air free.
Note that the wheel cylinder bleeders need to be clean and free flowing. Sometimes they will get packed with dirt, rust, etc and will need to be removed and cleaned in order to get a good flow of fluid out the valve and not have a lot of fluid flowing out the bleeder threads.
Hope this helps.
