No (Low) Rear Brake Pressure

[Miles]

Look here:

 

http://www.classicperform.com/TechBook/BrakeTroubleshoot.htm

[Twisted46]

Good link Miles,

 

I have worked on many brake systems before and the Datsun is definitely living up to it's reputation. I am still seeking out a local club member for a hands on look at things too.

[seattlejester]

I went through about 2 or 3 large bottles the first time I bled my system with no luck, and went to the parts store where one of the customers suggested this method and it worked pretty well. What worked was similar to what Jmortenson mentioned kind of a reverse bleed. Get two small capped container, drill or punch a hole in the top to snake some hose in, preferably clear, fill the jar with clean brake fluid until hose is submerged. With a short section of hose, set it above the bleed port on the brakes. Break both sets of bleeders loose in the rear or the front whichever you are bleeding and do long slow pumps, basically it also will pull fluid up backwards through the line. You will push air out which will bubble into the reservoir and it will pull fluid back up, if you have sufficient flow and a short enough line in the correct configuration you should find bubbles come out and the fluid going back in the caliper. Combined with fluid in the master it should mange to get enough fluid in the line to surround the bubble and use a bit of capillary action to move it along. Once the system is full it should bleed normally.

 

That is of course assuming you have enough throw in your system with the correct master, and assuming you have the correct length in the pushrod.

[seattlejester]

On 5/2/2019 at 12:51 PM, Miles said:

one more thing to check.

 

There is a push rod between the booster and the MC piston.  The rod is adjustable.  When you change MCs you have to adjust the push rod because the piston socket depth is different between the 7/8 MC and the 15/16 MC. Look it up.

 

If the push rod is  adjusted too long the brakes will lock up because the longer push rod prevents the piston from retracting all the way which blocks the fluid return port inside the MC.  This will result in your brakes locking up after  a few stops. If this happens while road testing just open the MC bleed screws and limp home and readjust the push rod.

 

If the push rod is too short you will experience a long pedal because the piston is not pushed far enough to fully engage the brakes. That is, the piston is not pushing enough fluid. Your brakes will feel mushy and you will experience a pedal with long travel before engaging the piston

 

Do a search for push rod adjustment. It is critical to properly functioning brakes and brake feel.

 

 

 

Question, this picture is in reference to the flange on the master correct? So between this and the booster there should be the aluminum spacer right?

[Miles]

2 hours ago, seattlejester said:

 

Question, this picture is in reference to the flange on the master correct? So between this and the booster there should be the aluminum spacer right?

 

You measure from the face of the aluminum spacer on the booster to the tip of the push rod.

 

 

So,  if you replaced the stock 7/8 MC with a 15/16  MC, which has a deeper socket,  you adjust the push rod out 0.53"as measured from from the face of the aluminum spacer.

 

The 0.53" is a starting point.  You may have to fine tune the push rod length by making small changes and then road test the car to make sure the rod is as close as possible to the bottom of the MC socket without pushing the piston too far inward which will cause the brakes to drag or lockup.

 

If the push rod is adjusted too long, the piston will block the return port in the MC and the brakes  lock up after 2 or 3 stops.  If this happens, open the MC bleeders to relieve the built up fluid pressure, limp home,  and adjust the push rod a little shorter etc etc.

 

If the pedal  moves too much before engaging the MC you will need to lengthen the push rod in small increments.

 

Note that you do not have to remove the MC to perform this adjustment. Just remove the mounting nuts and then pull the MC out and push it aside.

 

There is also a tool to measure push rod  length.

 

 

 

Edited May 7, 2019 by Miles

[seattlejester]

Thanks Miles, very very helpful!

[Twisted46]

I tried to get pictures through the wheels

[jhm]

Those appear to be the stock calipers; but if they were, the correct part numbered speed bleeder should've worked for you.

[NewZed]

The drawing in the Brake chapter allows you to see the line of force for the mechanical parts, all the way to the MC.  You need to get all of the play out of these parts before you start worrying about the hydraulics.  You're wasting brake fluid otherwise.  You're jumping around from booster to caliper to bleeder to NP valve.  Just follow the path from your foot to the MC piston.

[Twisted46]

Sorry for the long wait for an update, I have been really busy.

 

The speed bleeders from Russell do work. I won't go in to much detail but basically don't buy Dorman brand bleeders for your car, they look identical to the Russell but the nut is lower down and they bottom up on the nut before seating. I noticed the Russell's still had the thread locker on them and I must not have actually tried them. They fit.

So I then bled the brakes some more and things seemed to get better but not by much. I also managed to lock up the driver rear caliper now??? So I need to work on that more. I can now pump up the pedal indicating there is air in the system and it takes more effort.  I guess I just need to keep bleeding the brakes over and over. Ordered another set of bleeders so I can do all 4 corners by myself. It does stop thought.

 

I'll report back once I get more bleeding done,

[NewZed]

Sometimes people discover that the rear caliper bleed screw hole is not actually the highest point when installed after these conversions, because they're not designed for Z cars.  It's close but still has a spot for an air bubble.  They have to unbolt the caliper and rotate it so that the channel to the bleed port really is the highest point.  They bleed the brakes with the caliper loose, then rotate it back and bolt it down when they're done.  Be the bubble.

[Twisted46]

NewZed,

 

That is exactly what I ended up doing and it seemed to help some.

I have not tried reverse bleeding yet but that is the next step if nothing else works.

[Twisted46]

Okay so I now I have speed bleeders on all 4 corners and made some progress with pedal feel. I know there is air in the system because I can pump the pedal up in about 3 pumps. It is now just a question of getting it out.

[Twisted46]

So after messing with this countless more times I decided to order an Wilwood 1" MC. The only thing I did not try is reverse bleeding the brakes, mainly because I did not want to take out the speed bleeders just to have to put them back in. Also after further analysis I seem to have 1 pump that is low and then the rest will be higher at the same height with firm pedal. That makes me think it could but and undersized MC like Miles mentioned. 

 

I also plan to do larger front brakes at some point so I will need the upgrade. 

[Miles]

With the one inch MC you will notice that it takes more pedal force to stop the car. If you are going to larger calipers on the front then you will be all set.  Even with the one inch MC you may still notice longer pedal travel before the brakes engage after you install larger front calipers. It is kind of a domino effect when modifying brake systems.

Edited June 26, 2019 by Miles

[Twisted46]

Well the Wilwood MC is in and.... still the same problem. Now I am wondering if the problem is the POs crazy brake lines coils they put in the lines for the rear. I will try to get a picture later. 

[Miles]

5 minutes ago, Twisted46 said:

Well the Wilwood MC is in and.... still the same problem. Now I am wondering if the problem is the POs crazy brake lines coils they put in the lines for the rear. I will try to get a picture later. 

 

Coils?

[Twisted46]

Indeed, someone put some pig tails, 3 or 4 on the passenger and 6 or 7 on the drivers side near the rear drums (now calipers.) I will upload some pictures shortly. 

[720fast]

Yes my 77 had the same problem with my old lines i had to pump the peddel to blead because the vacuum pump would suck the liner inside the rubber lines shut and my peddle was soft because it had to blow them up. There are no coils in my lines i have the ss ones from the z store.

Edited July 11, 2019 by 720fast

[Twisted46]