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Hi. I'm pretty new to classic cars, so I hope someone here can help me out.

I just recently got a '72 Ranchero GT, 351C Engine. It needs work, but it works well, for the most part.

When the brakes are hit in a normal fashion (pressing them down gradually), they work great. However, when the brakes are pumped twice, the rear brakes lock up and the tires spin. I know old cars' brakes didn't have ABS in them, but I thought that the brakes used to lock up when stopping too quickly, making it where you were supposed to pump the brake. The right-rear tire seems to lock up sooner than the left does.

I had the brakes checked and the mechanic said that the brakes looked practically new, that I didn't need new ones, so I don't know what to do now.

Could someone help me out here? Thanks in advance.
 

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I have had similar problems. My '70's brake pedal would stick down after driving a few miles. My '71's rear brakes were getting too much boost. The '70 was a sticking master cylinder and the '71 was a defective booster. Other possible causes are the pushrod between the master cylinder and booster adjustment or a defective combination/proportioning valve. A new master cylinder/booster combination is pretty reasonably priced and would eliminate 3 of the 4 possible causes of your problem. Also, if your brakes perform differently after pumping them, you probably have air in the system.
 

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I had a 73 GT and had the same problem(s). The back of the RANCHERO is very light (cut-down SW body) and the OEM proportioning valve used is the SW model. On a slick surface, one had to be very careful of locking the rear brakes on a hard application.

First, make sure the foundation brakes are good. You may also have a defective front metering valve (front part of the combination valve) not allowing the front disc to apply enough pressure before the rear shoes come in. Ascertain that the caliper pistons are not frozen and the calipers slide easy. If the fronts don't work properly, the rear will have a heavier work load, leading to rear lockup. Check hand brake cables for sticking.

Stick your finger into the bottom of the MC reservoir and if you come out with muck, flush the system.

Diagnostics with a set of gauges and/or installation of an adjustable proportioning valve may be needed (and this calibrated on a skid pad).
 

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Jack up the rear and pump the brakes and see if the brakes are staying locked up for a short time. The fluid may not be relieving back to the mc fast enough. I have seen brake hoses going bad trap pressure and there will be a hose on the rearend.
 
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