Replaced master cylinder 3 times air in brake line after a few weeks only front right brake

Did air get into the master cylinder? If you can, try bleeding the master cylinder. The fact that you aren't getting any fluid out of the line suggests that the master cylinder isn't pumping fluid.

Master cylinders are (usually) on a split circuit system - that is, the front left brake is on the same line as the rear right. The front right brake is on the same circuit as the rear left.

A point to be aware of is when bleeding brakes/doing brake work and if the master cylinder runs dry, is that a seal inside the master cylinder can fail/twist over and not pressurise that particular line, even though it may pressurise the opposite brake line.

I'm in the UK. I have a 1998 Jeep Grand Cherokee 4 litre, which I am slowly rebuilding. Three weeks ago I fitted new front brake rotors, new front callipers and pads. I've been rebuilding cars for over 40 years now and have done countless brake overhauls.

.. and could I get a hard pedal after the job was completed? I must have bled the brakes three times a day for a week and couldn't get a brake pedal. I phoned a full time mechanic I know - he came and tried to bleed the brakes three times and couldn't get a hard pedal.

That's when you think 'The master cylinder was working before I started the job so it must be still working now'.

I fitted a brand new master cylinder and because some parts for older jeeps are hard to come by in the UK, I fitted a second hand proportioning valve. When I bled the brakes again the pedal became rock solid. Fixed.

You're probably fed up with your braking problem now ... and apparently this 'no fluid/brake pedal' is more common than you may think.

Try bleeding the master cylinder if you can. If you suspect that it isn't fully working - try an overhaul kit which is cheaper than a new master cylinder.

As for changing the proportioning valve .. that's your decision. My brakes now work though the truth is I don't actually know which was at fault as I changed both the master cylinder and proportioning valve at the same time. Maybe both were at fault, for whatever reason. I genuinely don't know.

The positive aspect is that you are doing your own work and know what you're doing. You're saving on labour charges.

Michael:

You must start bleeding the brakes at the wheel farthest from the master cylinder (usually the right rear), then the next farthest from the master cylinder, then the next, then the closest. If your master cylinder is at the left front of the car, start with the right rear, then the left rear, then the right front, then the left front. If you don't bleed the brakes in the correct order, you are just shifting the air in the lines from one line to another. Make sure that you close the bleeder before letting the brake pedal up, and the engine should not be running when you bleed the brakes... Make sure that the emergency brake is off. Make sure that the master cylinder does not run out of brake fluid at any time that you are bleeding the brakes.

AFTER BLEEDING THE MASTER CYLINDER ON THE BENCH, DID YOU BLEED IT IN THE CAR, IF NOT, OPEN THE LINE AT THE MASTER 3/4 TURN OFF, HAVE SOMEONE STEM ON THE BRAKES, DO IT SEVERAL TIMES UNTIL YOU SEE FLUID ON BOTH LINE NO AIR COMES OUT, THEN WITH THE KEY ON START FROM THE RIGHT REAR WHEEL , SOMEONE PUMPS AND HOLDS THE BRAKES AND YOU OPEN THE BLEEDER 3/4 TURN, THEN LEFT REAR , THEN RIGHT FRONT AND LAST LEFT FRONT, IF NO PRESSURE AFTER OPENING 4 OR 5 TIMES, OPEN THE SYSTEM BY THE BRAKE HOSE, BEFORE THE CALIPER, IF PRESURE THERE REPLACE THE HOSES. 14 YR OLD HOSES MAY BE INTERNALLY CRACKED. NOT ALLOWING FLUID TO GO THRU.

If you have replaced the line from the front to the back, there is a lot of air in the line. The proper way to bleed the brakes (for the average guy with no shop) is to;

#1, OPEN the bleeder screw,
#2 THEN have your friend push the brake pedal down fully to the floor and HOLD IT DOWN. Then you
#3 CLOSE the bleeder, so no air can enter the line from there,
#4 THEN have your friend let the pedal up after you have closed the bleeder screw.

Repeat this process untill you have fluid coming out of the bleeder screw. Remember, after your friend has pumped the brakes a few times, the fluid level in the Master Cylinder's reservior will
go down, and you need to refill it as the fluid gets pumped into the line. Do not let it go empty, or you will have to start all over again. Every time the brakes get pumped, the fluid will only go down the line a few inches, so you will have to do this Procedure many times, like 15 or so.

Once you get the fluid coming out of the bleeder screw, do the bleed procedure a couple more times, until there IS NO AIR left in the system and all that is coming out is fluid with NO bubbles.

You MUST do BOTH sides, right AND left of the rear, as there is only a single line to the rear, which splits into two above the axle, and now there will be air in both sides. Then, tighten the screw, top off the master cylinder, and you are done.

If you simply opened up the bleeder screw, and then pumped the brakes with it open the whole time, the master cylinder simply pulled air back into itself every time you let the brake pedal up, resulting in no fluid being pumped into the line.

Also, if the master cylinder ever was EMPTY, then you may have to bleed the cylinder itself. This is done by disconnecting the brake line (rear brake, the one you just had off) from the cylinder, and putting a hose from the line inlet in a small loop back into the master cylinder's reservior. Then, pump the brakes. The cylinder will then pump the fluid directly back into itself. The idea is that you do not let the master cylinder pull any air back into itself, from the line inlet. So, as the cylinder pumps out the air, the only thing it can get back in is FLUID. Do this until there are no more bubbles coming out of the line.

Then perform the above-mentioned Bleeding Procedure. If you do the Bleeding Procedure correctly, and the fluid level in the master cylinder does NOT go down, and you are still not getting fluid to the bleeder screw, then that will tell you that you have to bleed the Master Cylinder.

Good Luck To You.

Could be a couple of problems, bad master cylinder (not bench bled properly or bad seal out of the box, I have had a few like that), proportioning valve seal, brake line not tight/sealed properly sucking air into system,bad caliper. First of all, are you losing brake fluid along with your pedal? Need to know that first. If you are, try to find where you losing it from, you will find your problem. There can be absolutly NO leaks in a brake/hydraulic system. If not, check that all lines are tightly seated and nuts are quite tight using a line wrench to get proper torque. If so, you can replace the master cylinder again (take old one back to store for new one under warranty) and be sure to thoroughly bench bleed. When bench bleeding, take your time, do not push plunger in too quickly, use a SLOW steady push until ALL bubbles, no matter how small have been eliminated. Tapping on the bottom of the cylinder while bleeding will help remove trapped air. Even a very small amount of air will take forever to bleed out at the calipers. If cylinder/all calipers/lines appear to be right, I would suspect the proportioning valve is faulty. To find it, follow brake lines from master cylinder to where they go into the valve. Replace. Hope that helps.

The brake system bleeding procedure differs for ABS and non-ABS vehicles. The following procedure pertains only to non-ABS vehicles. For details on bleeding ABS equipped vehicles, refer to the ABS procedures later in this section.

WARNING Make sure the master cylinder contains clean DOT 3 brake fluid at all times during the procedure.

1. The master cylinder must be bled first if it is suspected of containing air. Bleed the master cylinder as follows:
   1. Position a container under the master cylinder to catch the brake fluid.
   2. Loosen the left front brake line (front upper port) at the master cylinder and allow the fluid to flow from the front port.
   3. Connect the line and tighten to 24 ft. lbs. (32 Nm).
   4. Have an assistant depress the brake pedal slowly one time and hold it down, while you loosen the front line to expel air from the master cylinder. Tighten the line, then release the brake pedal. Repeat until all air is removed from the master cylinder.
   5. Tighten the brake line to 24 ft. lbs. (32 Nm) when finished.
   6. Repeat these steps for the right front brake line (rear upper port) at the master cylinder.
      

WARNING Do not allow brake fluid to spill on or come in contact with the vehicle' finish, as it will remove the paint. In case of a spill, immediately flush the area with water.

1. If a single line or fitting was the only hydraulic line disconnected, then only the caliper(s) or wheel cylinder(s) affected by that line must be bled. If the master cylinder required bleeding, then all calipers and wheel cylinders must be bled in the proper sequence:
   1. Right rear
   2. Left front
   3. Left rear
   4. Right front
2. Bleed the individual calipers or wheel cylinders as follows:
   1. Place a suitable wrench over the bleeder screw and attach a clear plastic hose over the screw end.
   2. Submerge the other end in a transparent container of brake fluid.
   3. Loosen the bleed screw, then have an assistant apply the brake pedal slowly and hold it down. Close the bleed screw, then release the brake pedal. Repeat the sequence until all air is expelled from the caliper or cylinder.
   4. When finished, tighten the bleed screw to 97 inch lbs. (11 Nm) for the front, or 66 inch lbs. (7.5 Nm) for the rear.
3. Check the pedal for a hard feeling with the engine not running. If the pedal is soft, repeat the bleeding procedure until a firm pedal is obtained.

Fig. 1: Loosen the front brake line in order to bleed the master cylinder

Fig. 2: Connect a bleed hose from the bleed valve on the front caliper to a jar of brake fluid

Fig. 3: Always follow the lettered sequence when bleeding the hydraulic brake system

Hope this helps to solve it; remember to rate this answer.

blead from the furthest point forward... so , if your master is on the firewall on the drivers side.. rear right , rear left , right front and then left front...

Brother, it's just the fact that there is STILL air in your system. Just fill the master cylinder, open the left front line and let it run into a bottle to catch it for a bit until you know for sure all air is gone from that line. Then do the same with the right front. Then the left rear, then right rear. This is gravity bleeding, no pumping required. It's the only way to know for sure that each wheel cylinder and line is free of air. Only let the master cylinder drain down about half way each time the tighten each bleeder. Refill with the drained fluid. Be patient my friend, it takes time to do it right, but it works for me every time. Good day.

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.

to bleed the brake line somebody as to pump the brake pedal
3 to 4 times and somebody opens the bleeder on the brake caliper

starting by the right rear wheel, then the left rear wheel, then front right wheel anf finishing with the front left wheel bleeder.

to bleed the master cylinder is the same principle pump pedal 3,4 times
but this time open the line at the master cylinder starting with the rear hoses

hope it is helpful