The Ford Barn

mh_merc · 12-08-2018, 11:18 PM

'47 Merc; stock drum brakes with a Hydrovac booster; NEW master cyl., wheel cyls., rubber hoses, shoes.
PROBLEM: Pedal is very low when motor is cold, but gets higher and firmer as it warms up. Does this sound like a booster issue (it's the only thing that hasn't been touched)?

Kurt in NJ · 12-09-2018, 02:02 AM

You need to check that the engine vacuum to the booster the same or changes as the engine warms up ---if it stays the same look closewr at the booster, if the vacuum gets lower as the engine warms up look for engine problem---lower vacuum will make the pedal higher and firmer

mh_merc · 12-09-2018, 10:48 PM

Thanks Kurt - it's probably the booster; the engine is a good-running small-block with fewer than 30K mi. on it.

Flathead Fever · 12-10-2018, 04:01 PM

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We had Suburban's in our fleet at work. When you first started them, when the choke was on fast idle you had no vacuum assist from the booster. If you put them in gear you could barely stop them until the engine warmed up. It was scary. I have no idea how GM would have manufactured them this way?

If its like the cars I worked on there should be a vacuum check valve on the booster. If its holding vacuum you should have assistance when its cold. If it has one, shut off the engine and wait awhile. Then disconnect the check valve from the booster and see if it sounds like there was vacuum being held in the booster. If it does not have one maybe it is suppose to? If there is no vacuum then either the check valve is bad or the booster has a small leak.

12-08-2018, 11:18 PM	#1
mh_merc Member Join Date: Dec 2017 Posts: 32	Brake question '47 Merc; stock drum brakes with a Hydrovac booster; NEW master cyl., wheel cyls., rubber hoses, shoes. PROBLEM: Pedal is very low when motor is cold, but gets higher and firmer as it warms up. Does this sound like a booster issue (it's the only thing that hasn't been touched)?

12-09-2018, 02:02 AM	#2
Kurt in NJ Senior Member Join Date: May 2010 Location: on the Littlefield Posts: 6,159	Re: Brake question You need to check that the engine vacuum to the booster the same or changes as the engine warms up ---if it stays the same look closewr at the booster, if the vacuum gets lower as the engine warms up look for engine problem---lower vacuum will make the pedal higher and firmer

12-09-2018, 10:48 PM	#3
mh_merc Member Join Date: Dec 2017 Posts: 32	Re: Brake question Thanks Kurt - it's probably the booster; the engine is a good-running small-block with fewer than 30K mi. on it.

12-10-2018, 04:01 PM	#4
Flathead Fever Senior Member Join Date: May 2010 Location: Yucaipa, CA Posts: 1,095	Re: Brake question Sponsored Links (Register now to hide all advertisements) We had Suburban's in our fleet at work. When you first started them, when the choke was on fast idle you had no vacuum assist from the booster. If you put them in gear you could barely stop them until the engine warmed up. It was scary. I have no idea how GM would have manufactured them this way? If its like the cars I worked on there should be a vacuum check valve on the booster. If its holding vacuum you should have assistance when its cold. If it has one, shut off the engine and wait awhile. Then disconnect the check valve from the booster and see if it sounds like there was vacuum being held in the booster. If it does not have one maybe it is suppose to? If there is no vacuum then either the check valve is bad or the booster has a small leak.