Brake Issue - Am I on the right track?

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Jul 9, 2017
"New" (to me) vehicle that we took out for a maiden voyage. 1956 Chevy 210 with upgraded power brakes. Was hearing a "whistle" sound when driving but not under power that would go away when the brakes were applied. I also noticed a rough idle that lead me to believe I had developed a vacuum leak in the brake system. On the way home, the brakes started dragging to the point that I had to pull over and ended up towing the car home, although the brakes released by the time we got the trailer there. I was thinking it might be the seal between the booster and master cylinder but found that the booster check valve grommet was torn. I'll get a replacement today and hopefully this will solve my issue. My question, could a small vacuum leak cause pressure to not release from the brakes (lock up)?? Or is there something else I need to look at? Thanks!!
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I have never seen it but that doesn't mean much but I suppose in theory it could as it does assist in applying pressure to the cylinder, if it were binding inside it could prevent the proper return of the push rod. Test the booster on and off the car with a vacuum pump or just change it as that seems to be the problem part.
Your brake problem sounds like collapsed flexible brake lines to me. The whistle nay have no bearing on the brake problem.
There may well be two unrelated problems: 1. whistle, rough idle, and stiff brakes caused by vacuum leak 2. collapsed hose trapping hydraulic pressure at the brakes I don't think a failed booster will result in stuck brakes, but a failed hose sure will...
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Leaking booster will cause the "whistle". And in a car that old I would replace the soft lines if they haven't been done.
Usually bad hoses will result in a mushy pedal but they can separate inside when they are old. You don't say how the brakes were upgraded, does it still have drums and wheel cylinders or is it a disk brake setup? With drums corroded wheels cylinders and weak return springs can cause trouble, disc calipers usually don't get better if they are sicking just get worse. It could also be a metering valve sticking. Start at the beginning, you have a whistling and suspect the booster so that's where you start, if its defective or binding replace it first as its a known bad part then go on from there. Also check the push rod length as it is not original. When diagnosing things don't get the horse before the cart. As Astro said you may be dealing with multiple problems, its going to take a little patience.
To tell if there's a vacuum leak, pinch the hose shut and see if the whistling stops and idle improves. The master cylinder, booster, and brake pedal must be set up properly so that the piston in the master cylinder is free to come all the way back when the pedal is not pressed. Otherwise all 4 brakes will start to drag after a period of driving.
Thanks for the replies so far. Lots of good information. Initial plan is to replace the worn out grommet and then perform a vacuum test on the booster and go from there.
If there's a proportioning valve for the rear brakes that also has the emerg brake sensor switch, then you could have a jam holding pressure in the rear circuit. since the brakes were upgraded, it's worth looking at.
Hmmn How far from original is this Chevy? What is your intended use of this classic? Altered something in push rod dimension to eliminate free play and hold pressure is my guess. another 350/350 conversion?
We found that there is a leak in the master cylinder. Unfortunately, it's going to have to be replaced. Got one on order and will update after we make the swap.
1956? if its original there is no prop valve, no metering valve, 4 drums, and a single hydraulic circuit (I had a 57 bel air). Did these have oem power as an option? PS was a high-trim option IIRC, with the pump grafted onto the back of that 27Amp generator. I agree with the vacuum whistle, and it could be the cause. The booster should stay in full vacuum on both sides of the diaphram the whole time. When brakes are applied, pressure is introdcued into the high side of the booster, and the difference provides motive force. So, a leak could indicate the problem. Someone did mention above that the pushrods from the pedal to the booster and booster to the master cylinders must be calibrated properly, or it could also contribute. If not for that, I'd mention over-tightened shoes expanding and overheating and expanding more. Sort-of-related trick - if you have vacuum wipers that stick, we used to let the vacuum motor suck up a capful of brake fluid in the breather on the bottom of the motor. That would lubricate the leather wiper inside and get it going. Did this for yeeeeeaars. congrats on the neat truck! -meep
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