Author Topic: brake booster  (Read 835 times)

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Offline geraldt52

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Re: brake booster
« Reply #45 on: August 15, 2019, 11:55:36 am »
... pull a vacuum on the booster.A cheap Mity Vac will work...

I haven't actually tried it, but I think you're going to be there a very long time and have a very tired hand trying to pull a vacuum on a booster with a Mity Vac...especially repeatedly.  There's a lot of volume there...

Offline 69cobrajetrugae2

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Re: brake booster
« Reply #46 on: August 15, 2019, 12:57:16 pm »
The approach that I would use to isolate Dave's brake issue as an armchair quarterback is as follows.

Attach 4 1500 pound pressure gauges to the bleeder valves on all four corners and press on the brake pedal as hard as possible for 10 seconds with no vacuum in the booster and time and record the pressure readings.  Maximum pressure should be attained quickly and if not there may be a restriction in a line and/or a issue with the master cylinder or proportioning valve.  A designed volume of brake fluid must be displaced at a designed pressure.

Even without power assist the driver should be able to lock up the front wheels if the braking system is within specifications which would include asbestos pads and linings.

If an engine quits on a downhill grade at 70 MPH the vacuum reserve is good for 2 to 4 applications, after that the pedal angle of perhaps 4 to 1 will stop the car.  If 1000 psig at the master cylinder will lock up the front wheels then 800 psig will stop the car in the aforementioned example which will require 200 foot pounds of force on the brake pedal at a 4 to 1 pedal angle.

Imagine a leg press machine at gym with 200 pounds of weight on it.  Now imagine lifting the 200 pound weight just 2 inches off the stop with one leg.  Can't do it?  Now imagine traffic building at the bottom of the downhill grade and try again.

This paper explains how hydraulic pressure is created and managed using the brake pedal.

https://americanpowertrain.com/pedal-pusher-how-to-measure-and-adjust-your-clutch-pedal-ratio/

A booster is not required if the pedal angle is 8 to 10 to 1 and the pedal effort will be the same as a vacuum boosted or hydraulically boosted braking system   The problem with doing this is that there is not enough room under the hood to raise the master cylinder and/or the brake pedal will be close to the floor.

Here are some pages from the factory service manuals.  I've seen a foot pedal gauge like this,  a similar tool would be useful to isolate Dave's issue.



Offline geraldt52

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Re: brake booster
« Reply #47 on: August 15, 2019, 07:49:31 pm »
...These brakes were much better until recently and that led me to discover the m/c leaking into the booster. Hence the replacement of the m/c and booster...

My bolding above.  Based on this statement, I think the problem is being overanalyzed at this point.  If I'm understanding, there wasn't really a problem with the old booster, other than that the master cylinder had leaked brake fluid into it...which was only discovered because the power assist wasn't working that great with so little vacuum at idle.

The "no power assist" problem started with the NOS booster.  So, it's pretty likely that the problem is with the NOS booster...which isn't surprising at all, since it may have been laying on a shelf "deteriorating" for many years.  If I was ambitious I'd put the original booster back on and see if it still worked, then have it rebuilt if it did.  I'd return the NOS one for a refund.  If I was less ambitious I'd send old booster off to someone like Booster Dewey before I bothered to put it on.

Offline Dave R.

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Re: brake booster
« Reply #48 on: August 18, 2019, 05:37:55 pm »
...These brakes were much better until recently and that led me to discover the m/c leaking into the booster. Hence the replacement of the m/c and booster...

My bolding above.  Based on this statement, I think the problem is being overanalyzed at this point.  If I'm understanding, there wasn't really a problem with the old booster, other than that the master cylinder had leaked brake fluid into it...which was only discovered because the power assist wasn't working that great with so little vacuum at idle.

The "no power assist" problem started with the NOS booster.  So, it's pretty likely that the problem is with the NOS booster...which isn't surprising at all, since it may have been laying on a shelf "deteriorating" for many years.  If I was ambitious I'd put the original booster back on and see if it still worked, then have it rebuilt if it did.  I'd return the NOS one for a refund.  If I was less ambitious I'd send old booster off to someone like Booster Dewey before I bothered to put it on.

Let me clarify this. The loss of power assist occurred with the old booster and m/c. That led to finding the leaking m/c and replacement of both m/c and booster. That provided no improvement.    Dave R.
Currently:
1969 R code Mach 1 4 speed restoration. My occasional driver.
Formerly:
2000 GT 'vert 5 speed (her DD)
1998 plain jane 6 cyl. auto (her DD)
1974 fastback 4 cyl. auto ( her DD)
Seems to be a pattern here.