The Stovebolt Site Forums The Parking Lot Big Bolts Unusual Brake Issues on 1956 Chevrolet 6400

I am working on my grandpa's 1956 Chevrolet 6400 (2 ton) and I have replaced the engine due to a cracked head in the original. The new 250 ci needs a few modifications to take the hydrovac lines, but it can be done. In the meantime, without the hydrovac hoses connected, I should be able to bleed the brakes and hold pressure without those vacuum hoses being connected, correct?

The Problem: After bleeding the hydrovac and the wheel cylinders, farthest wheel cylinder to the closest and no cylinder has leaks (three wheel cylinders have been replaced, one serviced, and the driver's rear has not been touched yet, but has not shown issues), the master cylinder holds pressure, then about 5 to 10 seconds later (when the fluid level is to the threads, it squirts out fluid at the vent hole during that time) it loses all pressure and goes to the floor again. I suspect that it might be a master cylinder issue or an hydrovac issue. Can anyone lend any expertise on this?

Thanks for your help!

My understanding is once the hydrovac loses vacume you can use it a limited number of times before you lose most braking but when towing my truck home without the engine running I had a small amount of braking, maybe a inch of pedel, so if you have fluid in the master cylinder squirting out I would guess it's a master cylinder problem. Leaking wheel cylinders will run fluid down the wheels. I'm not sure if the hydrovac can leak fluid internally. Personally I would replace the master cylinder and all the slave cylinders if they show any sign of pitting internally. The brakes on these big trucks are marginal at best and losing your braking could be a disaster.

Vehicles equipped with hydrovac systems require a special bleeding sequence. Get a shop manual and read up on it to make sure your doing it right. Yes a Hydrovac can leak internally.

Im having real issues with my brake system, i cant get all the air out of the system. Ive tried everything i can do at this point in time without a pressure bleeder. Followed the sequence about 100 times been through tons of bottles of brake fluid and still only have 1" off the floor. With the shoes properly adjusted but it holds pressure, and stops better than i expected im just not comfortable with it taking that long to apply the brakes, the pedal gets higher by about 3" when you pump them a couple times. Ill be ditching my hydrovac, and master for a split system hydroboost, wilwood and other companies say that 1 & 1/8" bore masters provide enough volume for our big wheel cylinders. All you need after that is 2 residual check valves for front and rear.

The service manual shows first bleeder on hydrovac, then the one on top, then drivers rear wheel, passenger rear wheel drivers front wheel then passenger front wheel. Goodluck!

Sounds like you have it bled but the shoes are adjusted too loose, they should drag some when you turn the individual wheel by hand.

Chevy's till I d, the master cylinders aren't showing signs of leaking at all anymore now that most have been replaced or serviced.

Justhorsenround, I have the manual and will follow that procedure. I did it a few days ago, but I forgot to use the pneumatic bleeder on the hydrovac. I suppose that could have left some air in the system and caused problems.

BlackViking60, thank you! I will follow that again and use the pneumatic bleeder on all of them. I do need to upgrade a few of the bleeder valves though first.

sweepleader, I adjusted the passenger's side rear shoes down because the top shoe wasn't even touching the rod coming out of the rear cylinder (then I realized it was because the shoe was hung on something) and when reinstalling the drum, it slid on quite easily. The others are good and dragging a little (the only drum I haven't taken off is the driver's side rear... mainly because it took us two hours to break the lugs on the passenger's side with heat, but also because it's not showing signs of any issues). Would one loose side of shoes be enough to get it to hold pressure in the master cylinder only for 5 to 10 seconds and then cause it to lose all pressure?

I'm not an engineer, but I'd have to question that statement...the small 1-1/8" MC might have enough volume to engage the shoes, but when done how close to the floor is the pedal? Our big trucks have 1-1/4" or 1-1/2" bore MC's for a reason and putting small truck brake parts on them is a recipe for disaster IMHO.

If 1-1/8" MC are OK for your 2-ton then why did GM put 1-3/4" Dual Pot MC's in there 1980's Topkick's ???

Be safe!

Mike B

Sweep leader- they are adjusted just until i started to hear them drag they turn without much resistance but the shoes are touching the drums.

Mike b- to my understanding/ how it was explained to me, the factory uses 1 &1/4" bore mc 1 outlet to feed 6 wheel cylinders splitting that, where 1 &1/8 bore feeds just the front and rear seperately would be unnecessary to have a larger bore. Volume wouldnt be an issue with an 1/8" difference, then the hydroboost system will pick up the slack by adding all the pressure you would need and then some as explained to me. However from my own personal experiences taking my chevelle for example, smaller system yes, but i am able to go from 1 &1/8" bore mc, to a 7/8" mc and my brakes work just a little bit better. Thats 1/4 " difference in mc bore size with barely any change to my stopping distance or the functionality of my brakes or pedal height. ( i have tried this with all 3 sizes being 1 & 1/8", 1", and 7/8" mc's. The 7/8" mc was actually the most responsive to pedal input but braking distance still remained very similar.

Colby g- sorry for hijacking your thread, my apologies.

The adjusters need to be set so there is a lot of drag compared to a passenger car.

A 1.125" MC had only 81% of the displacement of a 1.250" MC. A 1.250" MC has only 69% of the displacement of a 1.500" MC

A 1.250" MC displaces 23% more and a 1.500" MC displaces 44% more than the next smaller example.

"Small" changes in bore size result in ever larger changes in displacement as the cylinder increase in size. This results in directly proportional longer pedal travel.

Combined with the slack in the brake adjustment, this results in low pedal height.

Brake effort will be affected inversely as the cylinder size is changed, brake effectiveness will not be affected as long as there is enough travel to apply the brake force necessary to stop.

sweepleader, I adjusted that one shoe on the rear passenger's side and the master cylinder didn't completely lose pressure like it was before. However, it still doesn't hold pressure at the top, 4 to 5" from the floor like after 2 to 4 pumps, but rather, after a wait, holds pressure about 1" from the floor.

Do you guys have any other recommendations?

After this and a new headlamp switch, she should be ready to roll!

One shoe likely won't do it, all shoes at all wheels need to be adjusted.

I agree with Mac and will add that the drag should be set to where it's on the edge of being hard to turn the wheel with one hand...heavy drag.

Mike B

sweepleader, I just adjusted that one because I had only adjusted that one before I took the drum off, so I was "putting it back" more or less. I know the rear shoes are adjusted quite tight, but I need to check the fronts now.

I do believe there might be a slight leak in the front driver's side wheel cylinder though, but I have another one that's a "brand new" original and I will swap it out soon.

I will keep y'all updated! Thanks for y'all's help!

I'm working on the brakes on my 1962 c60. The shop manual does a good job at describing the in depth bleeding procedure including bleeding the hydrovac and specifying the use of a pressure bleeder.

I made up a cheap pressure bleeder today using a hand pump weed sprayer. It was as simple as drilling and tapping a hole for a 0-20psi pressure gauge and adding a clear 3/8 hose in place of the sprayer wand. A metal plate with rubber gasket and a barbed fitting bolts to the top of the master to supply pressurized brake fluid to the systeem.

There are youtube videos out there showing how to diy make these units.

Colby G A way to tell if your close on shoe adjustment-drive not far feel the drums if a hot one bck off if cool tighten a little when all shoes are evened up then use your brkes more all drums should be warm after use. When you pull up on a little slope and let off truck will roll back easily stop it let off will roll back ! Just right !!!

fixite7, thank you for that! I haven't driven it yet, but I want to soon. I had to return to Georgia (the truck is in Texas with my dad and grandpa), so I will get back on it when I return in a few months.

I will keep y'all updated.

Thank y'all again!

Needing to pump master cylinder is sign of too small of volume - either master cylinder piston too small, or wheel cylinders adjusted too loose. When tightening the wheel cylinders, tighten them to the point where you can barely turn wheel with 2 hands, then back off until you can turn with one hand with difficulty. If you have an old master cylinder and your are pumping the pedal all the way to bleed brakes then you most likely ruined the seal on the master cylinder by having it travel over the unused rusty area. Also, if you converted to a dual master cylinder of lesser volume on each port, then you will not have enough fluid for the rear cylinders when the front cylinders bottom out. Most of the big trucks are designed for 80% braking in the rear. Most of dual cylinders deliver majority of fluid to front (assuming larger piston of disk brake). Not sure what your master is, but that needs to be kept in mind - I do know someone that plumbed it backwards on purpose for that reason and it seems to work well. My truck is a 54, so it might be quite different from yours. Also, fluid squirting up from master is not unusual and may not indicate a problem, depends on the master cylinder and its piston position when first depressed. I completely rebuilt the brakes on my 54 4100 back to stock, and everyone that has driven it is amazed by the braking power and ease. I had a lot of issues initially (wrong master cylinder, wrong wheel cylinders up front, pumped the master cylinder over rusty area and caused a leak, etc.....). Just some food for thought that helped me.

Have you had the hydrovac rebuilt? The diagram in them are leather and dry out and crack over time especially if mineral oil wasn't replaced on a regular basis.

The hydrovac may need to be rebuilt. The diagram in the unit is leather and can dry out oner time especially if the mineral oil wasn't changed on a regular basis.lots of other small parts that could be malfunctioning in the unit. As the name implies it's hydrolic over vacuum, fluid is being pushed thru the unit. If all your other brake components are functioning and shoes are properly adjusted all the leaves is the hydrovac.

Hi I found some assembly information regarding overhauling the master cylinder in the service manual.

Check out the attachment I posted there and under assembly on the top right hand side of the page it explains some common assembly problems and how they affect the master cylinder (braking). A distorted seal in particular.

Might help you to diagnose a master cylinder problem.