Why oh why are my brakes not working?? '73 620

[ccwade81]

1973 620 - Bought truck about 4 years ago and brakes would only slightly work when pedal was on the floor. I have since replaced master, booster, rear cylinders, rear shoes. Front shoes and drums replaced by PO. Bench bled master, set adjusters at all 4 corners, bled system. Pedal is soft and truck doesnt stop until pedal is bottomed out. Pulls about 18hg/in vacuum at idle, booster check valve still good. I'm at a complete loss, I just want to drive my truck! Please help!

[Stoffregen Motorsports]

How about blowing out the lines?

 

Bleeding the system can be done multiple ways. I usually start a dry system with one or all bleeders open then slowly pump the pedal to get fluid in the lines. I then let them gravity bleed for a half hour or so (making sure the reservoir does not run dry). The final bleed is done the old way - two people, one pumping three times slowly and saying "holding", the other at the wheel opening the bleeders saying "pump" and "hold". It's a tried and true system that works better than any kind of pressure or vacuum bleeder. I have both pressure and vacuum bleeders and I still think the old way is the best.

 

Sounds like you already have this done, but correct adjustment of the brake shoes is crucial before bleeding.

[wayno]

My first question would be how much play is there in the pedal before it starts pushing on the master cylinder sleeve/piston/plunger?

When you say soft this makes me think there is still air in the lines, does it have the single brake master or is it a dual brake master?

Edited December 21, 2019 by wayno

[datzenmike]

Only the 521 had the single master and maybe the 510. All 620s were dual master.

 

 

Push down on the brake pedal with your thumb. It should move about 1/16"-1/8". There has to be some play but not a lot.

 

From the top of pedal pad, down to bare floor should be almost 6 1/2". Is it close?

 

Look on the side of the master cylinder. It may say the size, and that should be 11/16". I may have been replaced with a smaller 5/8" ? This would make it mushy.

 

Have someone pump the brakes while you inspect the rubber flex lines from the frame to the wheels. The rear has a single one from the body to the differential. Do the hoses swell under pressure??

 

Brakes should be adjusted till the shoe just rubs the drum when turned by hand. Pump the brakes to center the shoes and check they are still just rubbing and adjust if not. Always loosen the e brake before doing the rears and re-set the e brake adjustment after.

[wayno]

OK, so all the 620s had dual masters, so when one has a dual master you have to bleed the rear circuit first(farthest away from the master), this rear circuit has to be bled first otherwise it will be very difficult to get the air out of it if not impossible if the fronts were bled first.

I have had this issue myself, in the end I had to open one of the front bleeders, stick a small hose on it and let the brake fluid drain into a jar while bleeding the rear brakes/circuit, once I was sure all the air was out rear brakes/circuit, I then re-bled the front brakes/circuit and i was good to go.

Sometimes you can drive it and use the brakes a few times and then have someone go thru the brake bleeding  ritual while you bleed/open the farthest rear drum away from the master, I have done this in the past to get every bit of air out of the rear brake lines, but I use a stick and do it myself.

[ccwade81]

Thanks so much for all the replies. Sounds like I just need to start over and triple check that everything is correctly adjusted and bleed the system again. Any merit to pulling the master and bleeding it again? It has not run dry during any work since installation.

[datzenmike]

You can buy pre-made brake lines. Get a 2' one with the same threads as the Datsun master and cut in half. Take the two outlet pipes off and thread the two lines onto the master and bend them up over and into the reservoirs. Pump the brakes and the fluid will be returned to the reservoir while you watch for bubbles. 

[ccwade81]

3 hours ago, datzenmike said:

You can buy pre-made brake lines. Get a 2' one with the same threads as the Datsun master and cut in half. Take the two outlet pipes off and thread the two lines onto the master and bend them up over and into the reservoirs. Pump the brakes and the fluid will be returned to the reservoir while you watch for bubbles. 

Great tip Mike, thanks!

[FrankRizzo]

 

Drum brake 620's have 3/4" master cylinders.

 

 

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Anybody remember the Master Cylinder from Felix the Cat cartoons?

 

[datzenmike]

 Yes 3/4" for dual masters. My '73 FSM lists them as 11/16" for the single/tandem..... and I should have read farther. My bad. The 3/4" was used until '78 when disc brakes were added and the master increased to 13/16".

 

 

I remember Crabby Appleton also.... no wait. That was Tom Terrific!