Brakes Dont Brake

[Researcher]

I have a 71 Datsun 510 that I bought as a mostly complete running project. It has some brake problems that Im kinda stumped on. The brakes basically dont engage until about 1/4 in from the floor. I originally thought no big deal the brakes just have some air in them. I started to bleed them and noticed the front driver side brakes where actually leaking. Took them apart and the brake cylinder looked old and rusty and had some pitting on it. I replaced the cylinder and seal and no more leaking. Awesome. Then I bled the brakes again and all looked good but the brake pedal still does the same thing it doesnt engage the brakes till the last 1/4'" of travel. It does have some after market brake parts setup that I am unfamiliar with. This was a Grass Valley Graveyard Special that was saved and semi rebuilt. 

 

https://www.flickr.com/photos/168829529@N03/shares/63QnS5

[datzenmike]

Front brake? You mean the disc brake caliper??? Rebuilt it or replaced it?? When replacing the caliper it's best to do both sides at the same time. Or do you mean the master cylinder??? ***

 

Better take a look at the rears. Loosen off the emergency brake and adjust the star wheel adjusters till the shoes just rub the wheel when you turn by hand. If the adjusters are frozen you'll have to remove them and free them up. If the rear brake cylinders are leaking you have to replace them with new. If the shoes are crap do them also. Stab the brakes several times to center the shoes and keep adjusting till there is no change and the shoes just barely rub. Do the other side and then set the emergency brake. Excessive travel of badly adjusted rear brakes use up a lot of brake pedal.

 

Might be a good idea to bleed the rears also if for nothing better than to flush new brake fluid through the lines.

 

*** If master was replaced... check and set the brake pedal play. Push the pedal rubber down with your thumb. It should drop about 1/8" before the master cylinder is met and the effort increases. More than this is simply a waste of pedal. The push rod to the master, connected to the pedal lever can be disconnected and screwed in or out to set the play. There has to be some play here.

 

*** Also a good idea to 'bench bleed' the master. You can do this on the car also. Get a short length of brake line with a proper threaded end. Install on the front output and bend the other so it loops back into the brake reservoir. Pump the brakes and expel any tapped air till only clear fluid flows. Now do the rears. When done, bleed the entire system fronts and rears. DO NOT LET THE RESERVOIRS RUN OUT OF FLUID WHILE DOING THIS.

 

 

I do see a brake proportioning valve beside the clutch master. Not really needed on a stock car.

[thisismatt]

Definitely need to bleed the whole system and adjust the rear drums. Need to know what you're dealing with to really help - what are the front calipers? Rears stock or disc or? What size is the master cylinder?

[datzenmike]

I see an O₂ sensor so KA or SR or something more than an L16. I hope the brakes were up-graded. Replace anything you find leaking or not working or bleeding is a waste of time.

 

29 minutes ago, thisismatt said:

Definitely need to bleed the whole system and adjust the rear drums. Need to know what you're dealing with to really help - what are the front calipers? Rears stock or disc or? What size is the master cylinder?

 

Yes not enough info provided here. If you have 300zx calipers front and back and a stock replacement master I don't doubt the pedal travel is excessive. Need to know more.

[Researcher]

Thanks for the input. It is a KA powered car and doesnt have upgraded brakes, stock discs in the front and drums on the rear. I was surprised when I saw it that way and I havent driven it that way. I replaced the piston on the driver side by the way, the actual brake piston. The previous owner had the master replaced. I did bleed the whole system the front and back. The proportioning valve was there when I got it. 

 

I was hoping you would say to check the adjustment of the back brakes,  I have very limited drum brake experience but I know I will manage just fine. Also thanks for the thought of checking for the master to be properly bled and for the info on how to adjust it as well. I will check back in when I get through with these few items and I do have plans to upgrade the brakes. 

 

My plan of attack is to bleed the master, check for proper engagement, then check the rear brakes for proper adjustment, bleed the whole system. Thanks again for the thoughts and ideas.

[Roadster-ka]

Stock 71 510 has a square adjusting shaft accessed on backside of backing plate. With wheel off the ground, turn clockwise as you look at it until wheel stops. Back off 1 or 2 detents. Wheel should be free. Repeat on opposite side.

Check every 6k.