Brake Booster Checkout

[Zedyone_kenobi]

I decided to run some diagnostics on my Z brakes booster and check valve system yesterday. The FSM states you should remove the check valve that runs from the manifold to the brake booster and check it for leak down and flow through. Well mine passed. Then it said to measure the leak down of the brake booster after the cars engine has been shut down. The process is described as such:

Install a vacuum gage after the check valve but before the brake booster. Test 1 has you run the engine until the vacuum gage reads 19.7 in Hg. Then turn off the engine and monitor the gage for 25 seconds or so and the vacuum drop should be less than .987 in Hg (or thereabouts). My car passed this as well. Then for test 2 it has you start the car get vacuum up to 19.7 in Hg again, and turn the engine off and press hard on the brakes. It says that you should not see a drop over .987 in Hg with the brakes pushed. Mine dropped all the way down to 15 in Hg.

I was wondering if this sounds right to you guys. If you push the brakes, wouldn’t that release the vacuum some how? I am wondering if I need a new brake booster diaphragm or just a whole new brake booster?

The reason I did this test was because my pedal always feels very hard and I was curious if I had any issues in my braking system. My brakes do work, and I can lock up the tires, but with considerable effort.

Any thoughts?

[KAL7467]

I have same problem. Lay under dash and push on the boot going into the booster. Mine leaks at that point when squeezed.I believe the po adjusted the rod to compensate for lack of vacuum. Not 100 percent sure though. Braked stop fine but are rock hard.

[Powerglide]

The pedal is hard because the rear wheel cylinder is stuck/sticking...

[ZCurves]

I decided to run some diagnostics on my Z brakes booster and check valve system yesterday. The FSM states you should remove the check valve that runs from the manifold to the brake booster and check it for leak down and flow through. Well mine passed. Then it said to measure the leak down of the brake booster after the cars engine has been shut down. The process is described as such: .....

.... My car passed this as well. Then for test 2 it has you start the car get vacuum up to 19.7 in Hg again, and turn the engine off and press hard on the brakes. It says that you should not see a drop over .987 in Hg with the brakes pushed. Mine dropped all the way down to 15 in Hg.

I was wondering if this sounds right to you guys. If you push the brakes, wouldn’t that release the vacuum some how? I am wondering if I need a new brake booster diaphragm or just a whole new brake booster?

The reason I did this test was because my pedal always feels very hard and I was curious if I had any issues in my braking system. My brakes do work, and I can lock up the tires, but with considerable effort.

Any thoughts?

Here are my thoughts -

As you probably already know, the Master Vac (Booster) is there to assist in building brake pressure to stop the car -making it easier on you and safer. From what you have described, it sounds like your Master Vac is just plain old and beginning to fatigue.

Consider this: The Brakes ARE the most important part of any car. You obviously have a lot of blood, sweat and $$$ invested too.

These cars are 30+ years old and I firmly believe that it would be prudent for everyone to really inspect their brake system -not just the linings, but ALL of it.

You have already performed a crucial test and it has explained why your pedal is hard and it takes considerable effort to lock up the rear.

So with all of that said, I would:

• Replace the Master Vac and the Vacuum check valve.
  
• Inspect Master Cyl. for leaks or Pressure Bleed (Pedal Fade).
  
• Clean the Proportioning Valve.
  
• Inspect the Calipers, check Pistons for smooth movement and leaks, Check Pads.
  
• Inspect the Rear Cylinders for smooth movement and leaks, Check Linings.
  
• Pressure Bleed and then Bleed it again.
  

NOTE: If your Brake Fluid is dark, then replace it. It is easy to do and will prolong the life of your brake system.

Properly maintained brakes will protect you and your precious Z.

I'll step off of my soap box now.

[Zedyone_kenobi]

Thanks Z curves. I totally respect what you have written. I have done most of what you are talking about, but not all of it. I will probably end up getting a rebuild master VAC. My check valve is in good working order according to the FSM. Not sure how to clean the proportioning valve, but I can read up on it. Calipers, pads, rotors, and shoes are all new. Just the wheel cylinders need to be looked at.

I will keep folks updated on my progress here.

[ZCurves]

Thanks Z curves. I totally respect what you have written. I have done most of what you are talking about, but not all of it. I will probably end up getting a rebuild master VAC. My check valve is in good working order according to the FSM. Not sure how to clean the proportioning valve, but I can read up on it. Calipers, pads, rotors, and shoes are all new. Just the wheel cylinders need to be looked at.

I will keep folks updated on my progress here.

I know that you are in Houston - you could get a rebuilt one from Advance for ~130.00.

The Proportioning Valve is not a big deal. It just is a PITA to loosen the hard lines. I can assist if you would like and help out with the Booster transplant - just PM me.

[Zedyone_kenobi]

I will give you a shout! My rear wheel cylinder should be in soon, after that I will shoot you a PM.

[ZCurves]

I will give you a shout! My rear wheel cylinder should be in soon, after that I will shoot you a PM.

Are you going to use later model cyl? I would, if you don't mind bending the hard lines a wee bit. No one is going to look at your wheel cylinders anyway.

Save some $$$.

[Zedyone_kenobi]

nope, already ordered the OEM part from courtesy. I am a stickler that way.

[Zedyone_kenobi]

UPDATE: the new rear wheel cylinder install went smoothly. No issue with dragging at all. Now I just ordered the master vac brake booster from Advance (120 bucks with 20 dollar Core! Not bad 91 dollars after core refund). It is a rebuilt one, and what I may do is keep my core, and just try to rebuild it once it is off the car. It would be a great learning experience.

Zcurves. I will post pics when I get it. I have to get some semigloss paint and a good replica sticker for it now.

[torker]

Anxious to hear if the new booster helps. I put the braided stainless lines on my sons car and rebuilt the rear wheel cyl. and bled a couple times and now the pedal is nice and firm and high but i seem to have lost the power assist. I am wondering if running the brakes in a new area of travel on the rod may have done something.

[FastWoman]

Zed, I'd just return the booster core. When I was an ambitious kid, I pulled my OK- (but not great-) condition '75 booster apart (per the FSM) and identified all the parts that needed replacement. I went with that list of parts to my local Nissan dealer (which was still "Datsun" only a few years earlier), and they looked at me like I was from Uzbekestan. No internal parts were available for it, despite what the FSM said. As I had paid them $80 for the FSM in the first place (the same FSM that instructed me how to rebuild the booster), I shamed them into selling me a new booster at wholesale. Anyway if that project couldn't be done in '83 (?), it surely can't be done now.