Installed new master cylinder and now getting low pressure to right rear and left front

Did you purge the master cylinder on the bench before installing, all line should be plugged except one and that one has a piece of tubing that goes back into the reservoir. The process has to continue until all the air is purged. Install, remove the plugs ( don;t worry about a small amount of fluid leaking as the lines are installed. I use plastic bottle filled about 1/3 full set on the ground, run a piece of tubing from bleeder valve into bottle. Install at right rear, crack the bleeder valve and pump the brake pedal slowly while a 2nd person keeps the reservoir full, they will also observe the bottle until all the air is purged, close bleeder and move the left rear and repeat, next the right front and finally the left front. Do not reuse the fluid is expelled. If this seems a little hard( It is really very easy) buy a mighty vacuum and pull the fluid thru the lines in the order of right rear, left rear, right front, left front. DO not let reservoir get low or you will spend all day starting over.

It is possible you got a bad master cylinder--even new parts can be bad. Are you following correct bleeding procedure? Starting with right rear, then left rear, right front then left front? Using a helper to pump the pedal slowly then hold to the floor as you open bleed screw, watch for bubbles then close it before pedal is released? Top up fluid often and insure it never gets low enough to suck in air? I once got a "new" master that had no check valve installed.

Sounds like a master cylinder problem.
Some cars have an equalizer valve that is part of the master cyl, and some are external. I'm thinking yours is part of the master cylinder. The front half of the cylinder may be applying pressure to those wheels and rear cylinder is not, or vice versa.

On these old manual brake systems without automatic adjusters you have to set the shoe clearance by turning the adjuster on each shoe out so the shoe is very close to the drum. If there is too much clearance between the shoe and drum the shoe will move as far as it can and never touch the drum, this will make a low or no pedal pressure.

Hi, bleeding your rear brakes with engine running, you should get enough pressure out of the lines to squirt 3-4 feet. In other words, good pressure. If all you get is a trickle, you probably have a bad master cylinder. Just because its full of fluid, does not mean the seals arent shot. If you have bled them in order, left rear,right rear,left front,right fornt and have bled them enough to get fresh fluid out of each one without achieving good solid pressure at all wheels, replace your master cylinder. There is a proportioning valve, but usually it will affect only one wheel when going out, usually the left rear.
Brake pedal height comes from the rear brakes, so if you have bad pressure there, you will never get a good pedal. Eventually with a bad master, your brakes will go completly out..Mike

This will take quite a few times to get there but,it will get there.Start with the right rear bleed it 6-8 times then check fluid in master cylinder afterwards go to the left rear and do the same. Then do the right front then left front and make sure the fluid in the master cylinder never runs low or you'll have to start all over.Keep repeating until pedal is up and firm

you may have to run the motor as you bleed the brakes so the abs hydraulic unit can purge any air that might be caught within the unit itself.did you bench bleed the master cylinder before installing it on the car.this can save time and reduce the possible chance of air entering the system.make sure you don`t allow the master cylinder to run low on brake fluid.i also assume that you are bleeding the brakes properly.Left front brake works with Right rear brake and Right front brake works with Left rear brake.this is how the brakes should be bleed.

Sounds like it is not completely bled. I would bench bleed the master cylinder first and then bleed to whole system again. The calipers take quite a bit of fluid and it sounds like you may have ingested air during the bleeding process. Start from the left rear and then right rear, then right front and then left front. Make sure you keep the master cylinder full during bleeding because it can **** air if low and you will not notice. It takes a while to do it correctly unless you have a pressure bleeder.

try disconnecting the front line, have someone else then step on the brake pedal and hold it down while you re-install the line. If that doesn't do it, you probably did get a defective master cylinder.

Always bleed the brake that is the furthest away from the master cylinder.

If your master cylinder is located on the drivers side, (Left), you would bleed the Right rear brake.
Then the Left rear brake, Right front brake, and finally the Left front brake.