Hey guys,

I did some brake work before my long trip and the brakes were good enough to make it but I am not sure what is causing my brakes to sink. If I stop at a light and hold the brake pedal enough to keep the truck from moving, the pedal will slowly sink. If I let up and reapply the brakes, the pedal will come back up but still sink and slower than before.

I recently replaced the front calipers and pads, vacuum pump and most if not all of the brake fluid from the bleeding process. When I replaced the passenger side caliper, the reservoir emptied before I could get the brake hose back on.

I bled the brakes several times. The last time, I had someone pump the brakes in the truck and I went from PR, DR, PF, ABS, DF. I didn't get any air bubbles coming out the second time.

MC and booster are of unknown age. MC has a Ford logo on it. I did not bench bleed the MC after running it out - not sure if I was supposed to or not.

 

Master cylinder is bleeding off, if you do not have a leak.

 

Agreed, master cylinder is failing.

 

Bad RABS HC module.

 

I have no leaks anywhere. The last 5000 miles I have put on the truck since the work and the reservoir is still full.

I'm guessing I should replace the booster at the same time?

 

I guess you can if you want, but If the booster is not leaking, I usually just replace the master cylinder.

 

Eaton,

It is called an internal leak. Most master cylinders have two O-rings inside them on a piston, that wear and fluid can bleed past them. That is why it happens when you are holding the pedal down, this happens inside the cylinder and no leaks will be evident and there is no loss of fluid. This is the bleeding off that Starmit referred to. You can rebuild it yourself with a cylinder hone and o-rings, or its time for a new master cylinder.

Good luck,

Jim

 

The booster and the cylinder are separate animals. You have a master cylinder problem. A bad booster will only make it more difficult to apply the brake. If you haven't noticed this, then yours is probably fine. If you are going to replace it you might want to consider the hydroboost conversion.

 

I understand the internal leak, I just thought I would mention that there are no external leaks. The only reason I mentioned replacing the booster is because I don't want to do the MC and then turn around the next week to do the booster.

Isn't hydroboost pretty expensive? I know a lot of people get parts from the salvage yard but I may not be so lucky.

 

Not sure on cost of hydroboost. I have heard the conversion is not too difficult. As far as your booster, I know that you are not putting any more strain on it by replacing your MC, like you would on an old pump with new injectors. If anything you are putting less, because you don't have to pump it at stoplights. Hydraulic pressure is hydraulic pressure, It take x amount to stop. Your booster should be fine if you haven't noticed any problems with increased effort to apply pressure at the pedal. I know I had a 66 Pontiac that the booster completely failed on, and I had to but both feet and my back into it to get it to stop, like trying to stop your truck with the engine off!

In this case I would say, if it ain't broke don't fix it. Those things can go a long time.

Mac.

 

If your master cylinder is not leaking inside the booster, you should be good. You said you are not loosing fluid, so this is not happening, and you can usually see it where the mc bolts on anyway. No one can say your booster will not go out next week, just as no one can say it won't last another 10 years, the main killer is a mc leaking into the booster.

 

OK, I'll keep the booster on there then.

What's the secret behind getting the lines off the booster or any other brake line connection? I have a flare wrench set but the connectors are on there so tight, it just wants to round off.

 

I don't know of any secrets, I have never rounded one with a proper line wrench, but I have had to remove some with vise grips that were rounded. If it has an adapter you will have to have a wrench on it too usually.

 

What are the sizes you used? I think my wrenches are all SAE.

 

Here are a few components I am thinking of getting to convert to hydroboost.

Cardone Remanufactured 52-7079 - Hydraulic Brake Booster | O'Reilly Auto Parts

Cardone Remanufactured 20-7238F - Power Steering Pump | O'Reilly Auto Parts

Master Pro P/S 71416 - Power Steering Hose | O'Reilly Auto Parts

Master Pro P/S 71442 - Power Steering Hose | O'Reilly Auto Parts