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When the engine is cold and the choke/fast idle is still on the power drum brakes in my 65 Cutlass are extremely touchy. When I was pulling into the garage I tried to feather the brakes to ease to a stop but instead the car stopped abruptly leaving short black marks on my new garage floor. They work fine after the choke is off. It's been a long time since I've had power drums but I don't remember them being this touchy when cold. Do I have a problem, if so what might it be?
My guess is contaminated brake shoes. Usually leaky/weeping wheel cylinders or possibly gear oil. And if they are contaminated I've never been able to clean them and end up replacing them.
Last edited by 66luvr; July 8th, 2019 at 04:41 PM.
My guess is contaminated brake shoes. Usually leaky/weeping wheel cylinders or possibly gear oil. And if they are contaminated I've never been able to clean them and end up replacing them.
Wouldn't that cause them to be grabby all the time? This only happens when the choke is partially on. Also there is no evidence of leaks.
Last edited by Cutlass Fan; July 8th, 2019 at 05:29 PM.
Hmm. Higher idle speed with the choke engaged = higher engine vacuum, therefore more "assist" from the vacuum booster. Not sure what that means, as in is something faulty?
I was thinking along the same lines as the vacuum and/or MC brake booster. I 'suppose' the test I'd try is this. Since it only happens when the vehicle is cold (first started), and the choke is open - before you start the car - disconnect the brake booster vacuum hose then test the brakes under the same scenario - do the brakes grab & lock up?
Hmm. Higher idle speed with the choke engaged = higher engine vacuum, therefore more "assist" from the vacuum booster.
Actually, the closed choke plate is what really causes the higher engine vacuum. Think about it - the engine is sucking against a carb inlet the is nearly blocked. As for why the brakes lock, how frequently do you drive the car? Rust can build up on the friction surfaces, which causes grabbing until it gets knocked off. It's also possible that the control valve on the booster is out of adjustment or sticking or something.
And, just to ensure we're clear on this point, you stated you had power drum brakes - from that statement I assume you have power drum/drum brakes (Front & Rear) and not power disc/drum brakes.
And, relative to Joe's statement (which is correct - the choke plate is closed) I suppose I should have stated when the choke is closed (which means a higher vacuum).
I'm all too often stating when the choke is 'open' implying you're applying choke when in fact the choke 'plate' is correctly closed - applying more vacuum.
Yes it is drum/drum. Norm it only happens when the choke is closed, I've never heard of adding choke as "opening" the choke. I will try disconnecting and I presume plugging the vacuum line to the booster.
Joe, I think I've driven it enough in the last couple weeks to knock of any winter rust. Where is the control valve on the booster.
Original single master or converted to tandem? Is the symptom new to the car or did it occur after any changes? One wheel not grabbing as much as the other three does lean toward a shoe material problem/contamination or adjustment. Any pulling under normal braking?
Good luck!!!
Last edited by Sugar Bear; July 9th, 2019 at 03:23 AM.
I will try disconnecting and I presume plugging the vacuum line to the booster.
Yes, after removing the vacuum hose to the brake booster, plug the vacuum hose leading to the engine so the engine doesn't lose vacuum. You don't have to plug the brake booster port where the vacuum hose was attached since we're now trying an attempt to rule out or identify the brake booster as suspect to the issue. The brake pedal will now mechanically engage the MC w/o the assistance of the brake booster since the linkage from the brake pedal runs directly through brake booster via a series of pistons & push rods into the MC.
Was your vehicle factory assembled w/ a brake booster (it came from the factory w/ power [drum/drum] brakes); or, was the brake booster (power brakes) an aftermarket upgrade at some point during its lifetime? The spring(s) inside the MC can weaken & deteriorate over time but so can the seals.
Here is an image where I added a brake booster, arrow for direction of travel and a couple circles identifying the port which must be closed (to apply brake pressure) or opened to relieve brake pressure and expansion of brake fluid when heated. It's a generic tandem MC but the operation remains essentially the same. It provides a talking point if the issue turns out to be the brake booster &/or the MC.
The control valve is on the firewall side of the booster, where the pushrod from the brake pedal enters the booster. It's the part labeled "poppet valve" in this diagram.
It is the original single resivor MC and I am pretty sure factory power brakes. There is no noticeable pulling to one side under normal braking, in fact I was impressed how well they do stop the car. It does seem to me that abruptness when cold has gotten worse.
I will try Norms suggestion as soon as I get a chance. Thanks for the diagram Joe.
July 8th, 2019, 04:32 PM
