The Stovebolt Site Forums The Shop Area Driveline '37 Chevy 1/2 Ton: Have to Pump Up Brake Pedal; Air in Lines?

The PO kept the original drum/shoe configuration on the front, but put drum/shoes from a ’69 Camaro on rear, and replaced original master cylinder with one from a ’54 Ford Club (kept it in the under floor configuration). Five years ago I replaced all the shoes and wheel cylinders and have driven less than 3,000 miles since then. It never has stopped very well but it was adequate.
However few months ago I adjusted one of my front wheel cylinders, and started noticing that I had to pump my brake pedal a few times to build pressure/stopping power. Noticed about the same time that I had a leaky master cylinder. So this week I replaced the MC with a new one (to match the ’54 Ford Club MC). I properly bench-bled the new master cylinder and bled the lines thoroughly (pushed about 20 oz of brake fluid thru) but still have the same problem with needing to pump the brakes to build pressure. No leaks I can find anywhere.
At the bottom of the master cylinder reservoir I have two holes to the piston cylinder; one about 3/16” diameter and the other just a pinhole; the larger hole is near the start of the piston stroke–the pinhole is toward the end of the stroke. Observing piston travel with the reservoir cap off, a few tiny bubbles come up from the pinhole on each return stroke. My brake lines hold only about 3 oz, so with proper bench bleeding and pushing 20 oz thru I can’t imagine I didn’t get all the air out of the system.
Any ideas why I still have to pump up the brakes?

Mike,

I'm currently doing a tandem MC conversion on my '36 Chevy pickup:

https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1196340/2.html

In researching that subject I've seen the condition you describe. I'm a geologist, not any kind of vehicle "expert". BUT I've read that one cause of the symptom you describe is fluid from wheel cylinders and/or calipers draining back to the master cylinder. In theory that can only happen when the MC is BELOW the level of the wheel cylinders and/or calipers. If that is what is happening to you, residual pressure valves will be a fix.

In my application I'm using a 1970s era Jeep drum/drum MC in the same under floor location as the original '36 Chevy MC. The information I've found concerning the need for "hold off" valves, proportioning valves and residual pressure valves is all over the internet, but unfortunately also very contradictory.

Maybe we could both work on this issue and the first guy who finds what actually works can tell the other.

Are the brake shoes adjusted properly? If they are too far away from the drums you would have to pump the brake pedal two or three times before the brake shoes make contact with the drums.

Rick

I believe the brakes are adjusted properly, but it's so hard to tell with the two star wheels on each cylinder. Over the weekend I will try adjusting the shoes tight against the drums and re-bleeding as some Stovebolters have suggested. I'll be back to let everyone know how that worked.

The under floor configuration was stock for so many years; do you know if they put PRVs on the lines back then?
Mike

Hello Mike. Sir, First thing, PRV is the wrong term. There is no PRV in our brake systems. Pressure Relief Valve or Pressure Reducing Valve is not a auto brake term that I am aware of. They are in our oil filter/pressure and A/C systems. The only valve you MAY need in a drum/drum is a 10LB Residual Valve. But probably not. Second, don't know what this means, you can't adjust cylinders. "I adjusted one of my front wheel cylinders". Assume you mean adjusted shoes with star wheel.
Next: If MC is below plane of wheel cylinders, install a 10 lb Residual valve.
Next: Rickmg's comment is VERY important. Adjust until wheel/tire can hardly be rotated. All four wheels. Drive around and see if it helps. If it does you can back off a little if you want.
Next: You can bleed at each cylinder all day long but it won't help if MC was not fully bled. After you drove your truck awhile it may have "introduced" air into the system thru a connection or two....or MC was not fully bled. Re tighten everything and rebleed MC on truck (messy). If you are seeing bubbles and not just fluid movement, then bubbles are not permissible per MIL-T-FP41c. (an old "skunk works" draftsman/designer will know what that means).
P.S. Not being picky about words but I have misinformed posters by misinterpreting words or abbreviations, please advise. I have also misinformed by just being wrong myself.

Excellent comment. Will be waiting to see results/hints obtained by this effort. As Yar says, we are all learning from each other.

Mike,

Here's a parallel discussion on Chevy Talk of the thread I started about my '36 Chevy pickup tandem MC conversion.

http://www.chevytalk.org/fusionbb/showtopic.php?fid/26/tid/341390/pid/2668609/post/last/#LAST

Some of this may be useful in what you're doing.

It sounds like he has the original huck brakes on the front.
If this is the case the adjusters are located on the wheel cylinders.
Fred

Thanks Fred, I didn't know that. 37 is out of my class. Sorry Mike.

Well I adjusted the shoes tight against the drums, re-bled the brakes, then backed the shoes off 7 clicks till just a slight drag on the drum and it seems to have worked. My pedal pressure is consistent now. The brakes still don't stop the truck nearly as well as I would like (some day I will convert to front disk brakes) but the problem seems solved for now. Thanks to all who chimed in on my issue.

Curious: What exactly is meant by, "...brakes still don't stop the truck as well as I would like...."?
Not trying to be a wise guy here, just trying to understand.
The Sparkster

Don't give up on the Hucks just yet. It may take a period of time on the road for the shoes to wear so that their arc will match that of the drums. Once the shoes are machined into shape by the drums from routine braking, you will need to readjust the brakes. You should then see a significant improvement in stopping power. In the '40s, the brake mechanics would grind new shoes to match the drums (until they contracted lung cancer from the airborne asbestos).
Carl

I would agree with Carl as I have experienced improvement over time with my '42 Huck brakes over time as they wear in. At first I could hardly stop on steep hills and now I can lock up the wheels if need be.

Regarding arcing the shoes to the drums, a brake shop in Phoenix (Phoenix Brake and Clutch) has done that for me twice. Once with my '50 COE and recently with my '37. The COE brakes were great at the outset and I haven't got the '37 on the road yet. So, to those replacing shoes I would recommend finding a local brake shop that still offers this service, for safety sake.

Kent

I don't do the seven clicks thing. I back off so there is very little drag. The tricky part is making sure both shoes are adjusted properly and not just one of the pair.

To Trukernix's comment: I replaced all the wheel cylinders and shoes 5 years ago (not a daily driver so I only have about 3,000 on those shoes now). I have the old Hucks on the front, and drum/shoes c. 1969 Camaro on rear. My front brakes have always been hard to adjust; though the shoes should be properly worn by now so that they fit the drums well, the drag when I rotate by hand is not uniform; I don't know if that's just the way those old brakes are, or if I have a set of slightly warped drums. I came to "7 clicks" by backing the fore and aft star wheels one click each until I could rotate the drum without too much effort (but see comment about uneven drag), so I think I've got it adjusted about as best as possible on both the front and rear shoe of each wheel.

To Sparkster's question: Despite plenty of miles to form the shoes to fit the drums well, and a brand new MC, it still takes me too long to stop for my liking (and for what I feel is safe); this is my first and only old vehicle so perhaps I'm spoiled by more modern vehicles with better braking technology, or maybe I'm still doing something wrong with my old brakes, or maybe the PO's design isn't adequate--he installed a MC from a '54 Ford Club, and designed an odd configuration where one port from the tee serves one wheel, and the other port serves the other three wheels. I just have the feeling that disk brakes would work better--but again, I'm not much of a mechanic, so it may be one of the above that is keeping me from getting the full stopping power of my brakes.

Mike, If you are still having braking issues with several miles on the Huck brakes (the shoes should match the drums by now), I suspect your issue would be related to your master cylinder.
Properly adjusted and arc-matched Huck brakes should stop on a dime (well maybe a quarter).

My experience with my '37 brakes agrees with Carl's comment above. The truck stops just fine with the original design brake system. The truck is only driven locally on county roads and city streets and typically at speeds less than 50 mph. Given that, the brakes do what I expect and need.
Good luck and keep us posted on how the problem is solved.

My comment on the "clicks" thing was actually referring to the manual although I don't know how many clicks they talked about. I have a 1938 Master and a 1951 GMC 1 Ton, both with the original style Huck brakes. Non of the drums give a perfectly even drag even after turning the drums. All it takes is a difference of a thou or so and the drums don't mount that accurately to the hubs. My brakes work really well with no vibration. The design of the original system takes into account the pedal leverage ratio, the master cylinder bore and the wheel cylinder bores, just among a few of the factors. You might want to check the bore size of the master as compared to the original.

Re Truckernix's comment on MC bore, does anyone know the diameter of the MC bore on the original '37 1/2 ton? My current master cylinder is from a '54 Ford and is 1-1/16".

Mike,

1937 Chevrolet Car & Truck Brake Specs

Mike
If it will help, some of the classic truck vendors sell MC replacements. I ordered one from Jim Carters for my '46 a few years ago. Since then I have seen them listed by BowTie Bits and Chevs of the 40's. These are 1"" bore, which would be correct for '37 1/2 ton.

Another thought, since the PO changed the MC to a ford mc, were by chance the brake lines and brass fittings changed, (re-sized) too? I know zip about any potential differences between the two, just wondering.

Tube size won't matter once the lines are full, volume of fluid moving through them is so little, the diameter is not a factor.
Pedal ratio and bore size will make a difference.

I agree with carl 100%, but to realize the wear-in scenario, the shoes should probably be tighter than 7 clicks loose to start. Wherein; wear-in will be much faster than waiting for 300 stops and you will improve stopping right away.
I have never admitted this in public before. I have always done my own brakes. Probably over a 60 times on over 30 vehicles. I have never turned the drums or machined the rotors. In keeping with Carl's comment, I not only let the curvature wear-in but I let the grooves in the drum/rotor wear-in to the shoes/pads. More surface area for friction! Or I'm cheap. Forgot which. Please do not follow this method. Keep the economy going. I will join a focus group. Thanks.

With the price of new Huck drums, I tend to do what bartamos suggests if the drums aren't grooved to severely. Yeah it will go through shoes faster, but shoes are easier on my wife's pocketbook than new drums.