Changing master cylinder and rear brake question

Topic: Changing master cylinder and rear brake question (Read 1427 times) previous topic - next topic

Changing master cylinder and rear brake question

September 16, 2013, 10:32:54 AM

I wasn't braking right so upon inspection I needed front brake pads but the rear shoes looked fine, however both rear wheel cylinders were leaking. I changed my front pads first while I waited on the wheel cylinders to arrive. The front brakes worked great and I had a lot more pedal. I changed the wheel cylinders a few days later which is where I'm currently stumped. After doing this I couldn't get a bleed at all in the rear. When you bleed the front brakes fluid comes out as it should and the pedal goes to the floor. When you try the rear only a few drops comes out and the pedal doesn't move. I tried gravity bleeding over the next few days which results in a slow drip similar to the amount I get when trying to bleed with the brake pushed. I collected a few onces on each side gravity bleeding and tried again to bleed with the same result. I get no movement at all on the rear brake bads or wheel cylinders while pushing the brake pedal either.
I checked the master cylinder and it appears that I'm not getting any fluid movement on one half of the resevoir. If I understand the mechanics correctly (which I'm not certain) then 1 side of the resevoir feeds 1 piston which goes to the back brakes and the other side feeds the other piston which goes to the front. I think I have a bad master cylinder at least I hope it's that simple and I've ordered a new one to come in this week which I hope to install Tues. Even if this isn't the issue the cost of the new master is very little and it comes with the bleed kit which I'd have to buy to test my old one.
Question for anyone who has done this.
I don't have a vise so I was thinking of mounting the master first then bleeding it using the bleed kit while having someone pump the pedal inside the car. Then taking off the bleed fittings and connecting the lines. This seems like it would be just as effective as pushing the piston with a rod on a vise.
Any thoughts on this would be appreciated, this is my first time installing a master cylinder.
Thanks: Charlie

Changing master cylinder and rear brake question

Reply #1 – September 16, 2013, 08:36:43 PM

Well you need to bench bleed it because you have to fill the balance port. I guess you can do it on the car with the engine running?? Question did you have brakes when you did the front and before removing the wheel cylinders?? Best way to bleed the system is with a syringe back bleed method. And yes one port is for the front and the other is for the rear. Best bet is to bench bleed it in a vice. If you do not have one find a friend that does. Basically you need to fill the balance port in the master. Other wise you will never get a PEDAL!!

I think you have air in the rear circuit. And changing the master is going to, mess you up further if you never bled a system before. TRY this disconnect the rear brake hose @ the master. Then hold your finger over the port. Have someone press the brake pedal down easily. This should spit out fluid and you will feel the pressure. Dont start changing things without testing. Try pumping the brakes then hold them then bleed the rears. You may have a collapsed rear flex hose but i doubt it. Normally you can crack the rear brake line @the master and see if it gravity bleeds. If it does the Proportioning valve may be blocked. Do some checking before breaking down the system and flooding it with AIR!!

Changing master cylinder and rear brake question

Reply #2 – September 16, 2013, 10:44:48 PM

Tom:
I was planning on just mounting the master to the car with the bleed tubes in place only as a means of securing it as I would in a vise. Then pumping the break without the car running to push the piston in and out as many times as I'd have to get all the air out. It seems like this would be identical to bench bleeding in a vise. Wouldn't this suffice to fill the balance port just as a normal bench bleed?
As far as rear breaks go, I really don't know if I had the rears working before I changed the wheel cylinders. I did have poor brakes which improved a great deal after changing the fronts but they still didn't feel right and this was before changing the wheel cylinders. I suspect that my rear brakes haven't been working at all but I can't say for certain.
I've bled brakes several times after changing pads and shoes but never ran into this. I've never bled a system after changing a master cylinder though.
I'll try your suggestions before changing the master out.
If it is a blocked proportioning valve how would you suggest I clear this? That valve is not available through ford, napa or autozone.

Changing master cylinder and rear brake question

Reply #3 – September 17, 2013, 05:33:08 AM

Not an issue as adjustable ones are available from WELLWOOD. But when bench bleeding you raise the front of the cylinder in the vice to get the balance chamber full. Then drop it to make sure the air is out. It is OK to do it on the car. Like you said what is the difference. Bit a lot of cylinders are not parallel to the ground. They are on angles. So i guess you can do it on your car because it is almost parallel to the ground on that car . Give it a shot. You most definitely could have a bad Master!!

Changing master cylinder and rear brake question

Reply #4 – September 18, 2013, 01:54:49 PM

Look at the soft line between the rear axle and the hard line on the car. These can swell over time and close up. Easy way to check is to disconnect the hard line from the body to the soft line and see if you get fluid. With the age of these cars it would not surprise me if replacing the front soft lines, MC, and rear soft line all help with braking. The seals on the rear wheel cylinders is what typically fails from age so it makes a guy wonder how the seals on the front calipers are doing.

If it were me I would be replacing everything on the braking system except the booster and the hard lines. It's some of the cheapest insurance and one of the most neglected systems on the car which just happens to be the most important. One other thing you might want to consider is a hardware kit for the rear brakes as well. This kit has all the springs, pins, and retainers which all wear over time.

Darren

Changing master cylinder and rear brake question

Reply #5 – September 19, 2013, 10:41:08 PM

I installed the master last night and bled it on the car with the bleed kit that came with it, that alone took almost an hour to get all the lines to where I couldn't see any bubbles at all.
I had left the rear hubs off which is how I could tell that I was getting no movement on the rear brake shoes. As soon as we began bleed the brakes with the new master the rear shoes moved as they should and I got a decent flow from the bleeders in the rear although not with as much presure as the bleeders in the front. I will check out the soft line as you suggest Darren. We did a thorough ugh bleed and drove it today. I have good pedal and it's braking much more normally so I'm pretty sure the issue was just the master, well along with everything else I changed anyway.
1 new issue that has came up is that my brake lights are blowing fuses now. I had taken off the brake switch thinking I needed to disconnect the pedal from the master cylinder rod and I think may have done something to the switch. It looks pretty easy to replace though.
Thanks for the help.
Charlie