620 Rear End Diagnosis

[thisismatt]

Well, what I said earlier still applies.  Didn't know you had a booster, so you need to pull the MC off the booster and make sure the rod between the booster and the MC isn't holding pressure on the MC piston.  It should have threaded adjustment and a jamb nut...make sure that hasn't moved.

[Randalla]

I have exactly 1/4" of free play. Does that seem sufficient Matt? Iceman, the reason for the boots not being in place is the pistons were pushed too far out when the pedal was applied. This caused the brakes to lock. I was able to get them back in after disconnecting the booster hose.  

 

Edited October 12, 2022 by Randalla

[thisismatt]

6 minutes ago, Randalla said:

I have exactly 1/4" of free play. Does that seem sufficient Matt?

 

Between the booster and the master cylinder (in other words, you unbolted the master to check)?

[Randalla]

No, I measured how far the pedal is depressed before it engages the booster. That's the way I learned to measure brake free-play.  

[datzenmike]

It should be a few mm like 1/16-1/8". The first 1/4" pedal travel nothing is happening. There has to be a small amount of play but certainly not 1/4".

[thisismatt]

I'm suggesting it's possible that the rod off the booster may need adjustment.

[thisismatt]

This end...MC side

[Ranman72]

yep what matt said the booster rod should not contact the master unless the brakes are applied 

maybe put a dab of paint on the end of the rod set the master on the bracket completely while the paint is wet then remove the master

verify no paint is on the master 

Edited October 12, 2022 by Ranman72

[Randalla]

So, I understand we're talking about two different things. I was adjusting "brake pedal" free play. I think what you're suggesting is there's a second consideration, the gap between the brake booster pin and where it mates with the master cylinder. Not sure how you would measure that though, since you can't actually see it. Am I wrong, or doesn't adjusting the nut on the yoke accomplish the same thing? As far as I know that's the only adjustment there is.   

[Ranman72]

 

the center push rod on the booster should also have free play if no free play when there is vacuum the brakes might be able to apply themselves 

in other words when you bolt the master to the booster if it touches and pushes even a little bit i believe it could cause your brakes to apply themselves without ever touching the pedal 

Edited October 12, 2022 by Ranman72

[thisismatt]

The rod on the booster has adjustment also

[datzenmike]

If original, the booster/master adjustment shouldn't change. Don't forget the booster may be at fault for something else and will be replaced shortly.

 

The pedal play, just push on pad with thumb till there is resistance. The movement should be about 1/16" or just enough that the pedal is not touching.

[Randalla]

Read today that not having a proportioning valve in a brake system with different brake types front and rear (front discs and rear drums) can cause rear brakes to  lockup. The factory did not install disc front brakes on 620s till 1978, yet my 1977 truck has discs (maybe previous owner install). I have been driving the truck in my neighborhood without locking up, since disabling the power brake booster, though braking is marginal at best. Does any of what I'm reading resonate with any of you???  

[datzenmike]

You're thinking of residual valves. Drum brake residual valves retain about 10-12 PSI in the brake circuit to take out any slack in the return springs and reduce pedal travel. Disc brake residual valves only need about 2 PSI or just enough for light pad contact to keep them clean. With 5 or 6 times the pressure needed a drum residual valve will potentially over heat a disc brake. 

 

 I don't think the 620 has a proportioning valve. Front/rear bias is designed into the master cylinder and the size of the rear brake cylinders.

[Randalla]

I think I may have diagnosed my problem. I bought and installed a new, Rock Auto master cylinder in  my 1977 620 pickup, which came with 4-wheel drum brakes from the factory. At some point before my purchase, my truck was fitted with disc brakes up front. I discovered there are different part numbers for a master cylinder for a truck with 4-wheel drum brakes vs. a truck with disc/drum brakes. I'm guessing the difference is probably to address the different pressures required to operate drum vs. disc brakes. At first, I thought maybe the part numbers were different for each year, but when I looked further on Rock Autos site, the part numbers for 1978 and 1979 620s (disc/drum) is the same and the part numbers for 1976 and 1977 620s (drum/drum) is the same. I guess the only way to know for sure is to buy a disc/drum master and bolt it up. Does this make sense???   

[datzenmike]

The drum brake master is 3/4" diameter, the disc brake master is 13/16". Either will work but the 3/4 (used with disc brakes) will have very slightly longer pedal travel to deliver the same volume fluid as the 13/16".

 

Raise one of the front wheels and try to spin the tire. If it has a drum residual valve it probably will be an effort. It should spin with finger pressure.

 

 

This is certainly not the answer to the rear brake problem. Disconnecting the booster seems to indicate the booster is. Have you made plans to replace it? .... you shouldn't be driving around with a compromised braking system.

 

Did you adjust the pedal play down from 1/4" to 1/16" yet????????? This gives maximum pedal and master travel.

 

[Randalla]

Mystery solved. Changing the master cylinder from the 1977 drum/drum version to the 1978/79 disc/drum version (different part numbers) has my brake system working properly now, with no lock up and plenty of braking assist. Though I don't know for a fact, I suspect there's something internal within the master cylinder that provides different pressures to the disc than drum wheel cylinders. Maybe a different diameter orifice or different valving. Anyway, I'm on the road now and everything seems to be working as it should. Thanks for all the spit balling, helping to figure this out.  

[datzenmike]

The disc brake master is larger diameter 13/16" than the all drum one, 3/4".

 

Remember that the front disc brake master will have a 2 PSI residual valve when it should be a 10-12 PSI for a drum. I would swap them from the old master. If you do, remember that the front circuit is to the rear of the master and marked F

 

Drum brakes require about 10-12 PSI to be kept in the circuit to keep all the slack out of the drum brake return springs. This reduced pedal travel before the shoes contact the drum.