Brakes sink Issue. Please Help!

[808countryside]

I have a Nissan 1985 720 extra cab truck. I posted a brake issue that i had about a month ago. I've changed the brake caliper, bled the brake system of how the manual states. Now! my brake pedal sinks while im at a stop light, or on a hill, etc. What could be the problem? Is it brake master cylinder or the Brake Booster? Im not to sure of what they both control, can anyone explain? It seems that my rubber brake hose is kind of wore or stretched, but i have no brake fluid lost or leaks. Could this be an issue? Has anyone have this issue. Please help.

[pnp720]

pressure is leaking past the seals in the master cylinder. If you initially have a firm pedal and then it slowly sinks down it is probably this happening. Replacing the master cylinder should stop your problem. Check to see if there are any leaks either in the calipers or rear wheel cylinders first a leak in the system can also cause this to happen.

[808countryside]

Thanks for the info. But i Don't see any fluid leaks from the front or the back. plus, my brake fluid level never goes down. Do you think the brake booster is bad or what does that control? but, i'll take a look into the brake master cylinder.

[captainzeros]

The master cylinder can leak internally past the piston. You'll never see the leak, but it'll give the symptoms of low pressure or a pedal sink like you're describing.

[808countryside]

I heard if you change the master cylinder that you have to bench bleed the master cylinder before installing. does anyone know what that means? and how do you do it? do you need any special tools?

[flinterman2000]

No special tools and its simple. Add fluid to the resovoir, with the cylinder in hand or in a vice use a screwdriver to move the piston. Once fluid comes out of the orifice for the brake lines, cover the holes and use the screwdriver again. There should be some resistance as the plunger in the cylinder builds pressure. Thats a bench bleed. Install on the truck, bleed again and you're done.

[captainzeros]

I like to run a tube from the bleeder valve back into the reservoir, then you can just pump the hell out of it without needing to worry about feeding it more brake fluid (don't be too vigorous though, the goal is to get air out and being too energetic tends to make bubbles). Forewarning though, it's sort of a messy process, I haven't found any method that doesn't result in spilling some brake fluid somewhere along the line.

[flinterman2000]

Well thanks Captain. I think I will try that next time I have to bleed. Don't know why I never thought of that. Makes it a one man job.

[captainzeros]

Well, that doesn't make bleeding the whole brake system a one man job, it only makes bleeding the master cylinder itself a one man job (which it is to begin with). Bleeding the whole system by yourself you got a few options, generally being buying a power bleeder (So I hear they make the job a lot easier, but can also make it even more of a pain if you get a cheap one and it leaks. No personal experience with power bleeders myself) or trying to gravity bleed the system which I don't know if you can do with these trucks effectively or not (which involves taking the cap off the master cylinder, opening a bleeder valve, and letting gravity drip the brake fluid from the top of the system to the bottom over the course of an hour or so while you drink a beer. Keep the MC topped off or you have to start over, naturally).

The key to bleeding is pushing any air in the system out one end. Without a power bleeder to push it backwards out the master cylinder you have to push it from the MC to the wheels. "Bench Bleeding" the master cylinder pushes any air out of the master cylinder out it's bleeder valve so there isn't any air trapped in it, but doesn't have any effect on any of the lines downstream of it towards the wheels.

(edit, sorry if any of this is rambling or sounds patronizing, I just woke up and my thoughts aren't all coherent yet just wanna make sure your brakes work perfect)

[dogred]

Just remember to start your brake bleeding at the Load sensing valve, then the passenger rear, driver rear, passenger front, driver front.

[flinterman2000]

Its basically the same for right hand drive. Bleed the furthest first and the closest last.