Still Having Power Brake Issues

[jerryacheson]

I need your help to diagnose a power brake problem. I have a 62 Electra that had power brakes with a single master cylinder and drums all around. I had the brakes re-done, including a new master cylinder. After the repair, I noted that the brake booster wasn't working. I did the 'test' (i.e., engine off, foot on brake, start car) and the pedal pushed toward me as it was suppose to. I scratched my head and decided to put a new booster and dual master cylinder on it anyway. The booster is from Summit (8") and the master cylinder is a standard GM unit that has the same push rod style as the booster. The booster calls for 18" of vacuum which is what I have off the intake. I plumed in the vacuum canister that was in the car. The engine performs fine which would indicate to me that there is no vacuum leak. However, I still don't have power brakes. I do have manual brakes but it takes a lot of pressure to stop that big ragtop. The 'test' doesn't work on this booster. In fact, I think the pedal goes down a little bit when the car starts. What are your thoughts on my problem?

[Guest brh]

Don't know much about the booster, but I do like to troubleshoot. In 62 the dual master was not avail. Is it possible, something was not done correctly and the only thing that is stopping the car are the fronts or rears. Reason I ask is I know either a pressure adjustment has to be made or something along those lines for the dual to act as the single did. I remember researching a conversion for my car and finding out all sorts of things that need to be done for the brakes to act correctly. If you pull your drums you may find only fronts or rears are working. Just a thought.

[jerryacheson]

That's worth checking. It's my understanding that you don't need to use a proportioning valve if the fronts and rears are both drum.

[Guest brh]

I think ya might want to check into that. If I recall in 67 when the dual cyl first came out the size of the front wheel cylinders were larger than the back. Now I don't know if this was because of the dual cyl or a pressure issue.

[sixtytwopost]

Just a thought? But the threaded rod on the end of the brake booster that adust the throw.Did you try adjusting it at all.If the eyelet is threaded in towards the firewall to much.That will make the pedal have too much freeplay before the brakes come on.

[NTX5467]

Both the size of the piston in the master cylinder and also in the wheel cylinders can vary. Such variances are how the hydraulic "leverage" is utilized. Front brakes would typically have the larger of the wheel cylinders as the fronts do most of the braking, so the rears would be smaller than the fronts to reduce the tendency to lock up too soon (due to weight transfer "unloading" the rear wheels during a fast stop). Also consider the (usually) smaller diamater and narrower brake linings on the rear (as compared to the front) as other ways that front/rear brake bias was designed into 4 wheel drum brake systems.

In a dual master cylinder, the piston diameters for the front and rear can also vary a little.

No proportioning valves were used until disc brakes came out. What you might see and suspect it's a proportioning valve is the brake switch pressure sensor. This is what turns on the "BRAKE" light in the instrument panel when a loss of brake fluid (and resultant loss of pressure in either the front or rear systems) takes place--not a proportioning valve in the earlier drum brake systems as the "proportioning" was done via the master cylinder piston ("dual" circuit master cylinders) sizing and/or wheel cylinder piston sizing (front/rear).

In later years, there were brake switch blocks and also proportioning valves, later combined into what GM called a "combination valve" that had the brake pressure differential (fluid loss) switch and a separate front/rear proportioning valve in the same casting.

Just some thoughts,

NTX5467

[Guest brh]

I know I am getting off the "beaten path" here but I just want to understand NTX. What you are saying is because of the mechanics of the shoes themselves, it is possible to go straight from a single cyl to a dual? Just want to know as I may want to reconsider a dual on my 65. I have lost brakes twice. Try stopping that puppy on emergancy brakes all the way home, it will make ya think twice about the value of those points.

[jerryacheson]

This has been the strangest thing. I started 2 years ago with a perfectly fine car. I bought shoes, mc rebuild kit, new wheel cylinders and hardware and took it to a local shop to have the brakes rebuilt. While I've always had a firm pedal, it took a lot of leg pressure to stop the car. I always attributed this to the booster. As indicated above, I bought a universal 8" booster from Summitt and converted to dual mc. No change. I then bought a 67 Buick Electra 10.5" booster and mc and put those on (bolted right up). No change. I just put new shoes on from NAPA and the brakes are fine now. I guess I just had a bad set of shoes. All that money wasted on boosters and master cylinders. Oh well; I can't take it with me. I think my troubles are over on this project now. Now I can enjoy the car again. Thanks for all your help.

[Bhigdog]

The same thing happened to me with my 57. When I started the restoration the car came with some x-tra parts including front and rear new shoes. When the car was finished it wouldn't stop worth a damn. I attributed it to the shoes not being broken in. After a few miles with no improvement I sent the rebuilt booster back to the rebuilder who tested and returned it as having no problems. As a last resort I put new NAPA shoes on all 4 wheels. Problem solved....Bob

[Guest Jeff_Nevada_City]

Just joined and perhaps anyone involved with the original thread is still around, but here goes. I have a Ford F100 CrewCab custom ordered in '64 from a coach builder. It has Buick Electra booster and brake master. The F100 has drums front and back. I replaced the single with a '67 dual master and it bolted right up (which answers a previous question!). It feels like it bottoms out within the master before I come to a full stop (not safe). New wheel cylinders, new soft lines, shoes adjusted, no leaks, bled profusely. Still the same problem.

How much pre-play should the adjustment rod push the master when it is bolted to the booster? Does the master have the same diameter piston for both front and back brakes? (if different diameters a proportioning valve may be needed)

Any other ideas on what to check?