1970 Blazer Brake Question

[Chad22873]

Well its been several years but I am back. I started my frame off in 2016 and got about 80% complete and then quit for a few years. Got back to it last fall and am now about 95% done. The only real problem I have now is brakes. I dont have any. Everything in the brake system is brand new. Here is what I have done so far...Bench bled the master, then vacuum bled the system at each wheel about 5 times. Pedal still goes to the floor. Power bled the system twice. Pedal still goes to the floor. Replaced the new master cylinder with another new one. Of course, bench bled it first. Then power bled the system, 4 times and pedal still goes to the floor. I get no air at any of the wheels. My rear brake pads are adjusted correctly. When I push the pedal, all 4 wheels do engage but not tight enough to have brakes and pedal still goes to the floor. I have done this my whole life and I just cant seem to figure it out. I thought maybe you guys might have some ideas. Its a simple system but I cant make it work.

[1970cstblazer]

Are you using DOT 3/DOT 4 or DOT 5 fluid?

[72ironmike]

I apologize in advance because this probably isn't much help. Is linkage from brake pedal to master engaging ? Does the pedal pump up from floor when you bleed brakes ? You say you get no air but do you get a steady stream of fluid from the wheel cylinders ? You know it is gonna be something simple ! You have done way to much work for it not to be.

[Chad22873]

Dot3

[Chad22873]

The pedal comes back up. The brakes engage at all 4 wheels when the pedal is pushed down. They just dont engage tightly enough to stop. I do get a steady stream of fluid at all 4 wheels. The last time I bled them yesterday I actually put a gallon of brake fluid in the power bleeder and let each wheel run until it filled up a 16 oz cup. No air the whole time it ran at any wheel.

[Getter-Done]

Do you still have the Original master cylinder for comparison?

[notsolo]

Did 70 have Brake booster? Rod length between master and booster could be too short. Prop valve?

[72ironmike]

Originally Posted by notsolo

Did 70 have Brake booster? Rod length between master and booster could be too short. Prop valve?

It sounds like this is all it could be.

[Chad22873]

Unfortunately no.

[Chad22873]

It did have a booster on it when I got it and I replaced it with a new one. Perhaps the original did not and if I am buying an original master maybe thats an issue.

[Chad22873]

It does have a prop valve which is also new. I have rolled that around in my head many times but cant seem to think of a scenario where it would be the problem.

[Chad22873]

So even if the rod is touching the master when I bolt the master up, you guys think that perhaps the rod isnt long enough to complete a full stroke?
If so, does anyone have any idea how long the rod should be?

[notsolo]

If the rod is seated in the booster and in contact with the dimple in the master cylinder, that's not it. Replacement booster rod same length to the pedal as original?

[Chad22873]

Really dont know since I replaced all of that about 5 years ago now. Got it up and running and ready to drive in the last month. Only thing left is the back seat. So I went to drive it and couldnt get brakes to work. Thats when I put the second new master on and still same problems. I will pull the master back in a few min and check the rod. If the rod looks good then I may try the power bleed one more time. Although I have my doubts about the rod. The pedal is at the top and feels like its engaging the master as soon as you press on it.

[hemi43]

Disconnect the lines at the master and plug them. This will determine if your linkage and master is working or not. If you get a stiff pedal, then you'll know the issue is with air traped in the lines or brake cylinders.
Using DOT 3/4/5 makes no difference
You didn't specify if you have front disc or drums.
What is the bore size of the master?

On some masters, you can look down inside the bowls see the pistons move if you look carefully into the small orifices.

Another thing you can try is have someone push lightly on the pedal while you look at the brake fluid in the master. You should see the fluid move a bit when the pedal is first applied because the piston will push a bit of brake fluid back into the reservoir before you get pressure build up. It does this because of the small orifice I mentioned above.

[Chad22873]

Well I pulled the master back and what do you know...the push rod was not long enough. An inch short. Honestly I am surprised that I missed that. Evidently, when I replaced everything I just never thought to look. I had adjusted everything up nicely on the other side of the booster but never looked at the push rod on the other side. I made a make shift rod and stuck in there just to see if it that fixed the problem and it did. Thanks to all for replying. Guess I need to order and adjustable push rod.

[notsolo]

Look under the bench thats where everything I want ends up, I think it should be about 2 inches long. Glad you found the problem.

[72ironmike]

Awesome !!!

[1970cstblazer]

Originally Posted by hemi43

Using DOT 3/4/5 makes no difference
.

BULL!

DOT 5 is a ***** to get microscopic air bubbles out of. I had a heck of a time bleeding a system on a friends 1973 Trans Am. We could not get a very good pedal feel, and eventually flushed the system, replaced all of the rubber and went with DOT4. Problem solved....

[hemi43]

Originally Posted by 1970cstblazer

BULL!

DOT 5 is a ***** to get microscopic air bubbles out of. I had a heck of a time bleeding a system on a friends 1973 Trans Am. We could not get a very good pedal feel, and eventually flushed the system, replaced all of the rubber and went with DOT4. Problem solved....

As far as the problem the OP had, I said DOT 3/4/5 would make no difference and I stand by that statement.

The main difference between DOT 3 and 4 is the boiling point.
Dot 5 is silicone brake fluid and should NEVER be used for daily driven street vehicles, but I guess you learned the hard way and figured that out.