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Adminstrator And Sheet Metal Junkie
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Discussion Starter · #1 ·
Hi Guys
Need some help on some braking questions. Usually brakes are a no brainer ! ! However the past few weeks I had an old customer stop in with some upgrade to his brakes, from standard to power. Well honestly the standard where a better stay.... :rolleyes:
Here's what I did :
Changed standard master to a disc / disc master with a one inch bore.
7 inch dual diaphragm booster.
Also two 2lb residual valves, one for the front and one for the rear.
Then with SSBC's remote vacuum pump and accessory.

Now, we went threw a host of parts from SSBC because they didn't work properly. But finally got in touch with a good tech guy at SSBC and got the parts thing straightened out.
Now the brakes are still giving noting but problems. First I blend them, starting from the bench bled master cylinder and on to the right rear , left rear, right front left front.
Test drove the 34 pickup and it had brakes but the pedal wasn't what I called great.
So I called a friend and had him bleed it and he found no air, and brakes were no different.
So customer takes the truck and says, hey there not bad but here's what there doing.
He was running threw town where there was a red light at every corner. Now by about the last stop he said the the brakes were like a brick and the remote pump would not turn on.
Anyway, he gets the truck home and its sitting in his shop for about half hour and the vacuum pump turns on and he has brakes. Not super power brakes, just good brakes.
So he ask me something ! ! He said where dose the vacuum from the booster go after you step on the pedal ?
Anyway, why the nightmare on the brakes. Now the good part is I built this truck from scratch and the owner knows I'm competent but I'm whooped on what else to do.
I thought maybe taking the residual valves back off to see maybe if they are causing interference.
Or could it be the master ? Is the fluid running by the piston ? If so, how is it bleeding out ?
I've had some hot rods give me a time bleeding out, but when done, they have brakes ! !
What's your input :) :)
 

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Is the master under the floor Steve? Or is it lower than the calipers? If it's not lower than the calipers, then you don't need residual valves. Usually those are to keep the system from flowing back into the master and creating a soft pedal feel, and having to double pump the brakes to stop.
This could also be causing the issue with the pedal pumping up too firm, and not staying at the same pressure. The vacuum pump should only come on occasionally, if everything is OK. A vacuum gauge in the system would tell you a lot more about how it's all working. I had one on a car with a vacuum pump, and a reservoir, and mounted it on the reservoir to keep an eye on how much vacuum the system had. If the pump wont keep up, or run often enough, it might be a pressure switch problem, and may need to be replaced or adjusted to keep the vacuum up around 15 inches.
 

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Adminstrator And Sheet Metal Junkie
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Discussion Starter · #3 ·
Hi Vall
Yes the cylinder is floor mount, or I should say mounted to the frame. Which dose make the calipers higher.
Now as far as the pump, it has a vacuum switch that only turns the pump on at when it gets to about 16 pounds and shuts off at 24 pounds. The vacuum switch goes threw a relay and until the other day with his cruise threw town. Now from what Randy says, they worked ok and the pump worked flawless ti he had to work them threw town and really pump the brakes often.
Now I've had some that I've built and that I've put on the frame that had no issues without the residual valves. I told randy last week I'd like to try the system without them.
My Willys had CNC masters and no residual and had excellent brakes, the 34 Chevy has frame mount with GM master, and no residuals, and work better then Randy's with power.
It has to be one or more of the components.
By the way, where dose the vacuum go after you step on the brake. :)
 

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The vacuum doesn't really go anywhere. It stays in the system, and when the pedal is released the booster diaphragm returns to it's normal position and the vacuum is ready to do it's job again. An under floor can work without residual valves if everything is engineered to work together, so the master and calipers match, and you have enough volume in the master to move the calipers far enough, and brake pad to rotor clearances are small enough to not have to move the pistons too far.
The issue of him having the brakes fade away with numerous stops kinda indicates the vacuum pump isn't keeping the vacuum high enough, or retaining enough "volume" of vacuum to allow multiple stops without depleting the vacuum. I would still be questioning whether the pump is cycling properly, and retaining the 16-24 inch of vacuum it's designed to maintain. Should be able to put a T in the line, and see what range it runs at, and with a long enough hose you could even keep the gauge inside the car to watch the vacuum readings as you drive it. I have a small universal vacuum gauge in my tuneup kit, and I use it this way when I'm tuning an engine, and want to drive and watch vacuum during operation.
Mine is like this one:

$15 at Summit.
http://www.summitracing.com/parts/wmr-w80594
 

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Adminstrator And Sheet Metal Junkie
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Discussion Starter · #5 ·
Hi Vall
I have a about the same Vacuum gauge but a lot older. :) I tested the pump before I installed it and it ran a 30 pounds. I checked this because it was our second pump, which the first one only drew 20.
I am also questioning the volume, but many discussions with SSBC they keep tell myself and Randy ( he had purchased all the parts ) that the master was fine with the brakes he had. I at one time mentioned maybe to step up to 1 1/8 bore but I know volume may be more of an issue.
A ( T ) in the line and a vacuum gauge in the compartment would be a good next step to take. This would either find or eliminate one of the components.
I almost néed to take the truck for the weekend and test drive it myself. :)
 

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Hi Vall
I have a about the same Vacuum gauge but a lot older. :) :)
Probably as old as mine Steve! I got my tune up kit (with the vacuum gauge) from Monkey Wards when I turned 16 in 1966. The timing light eventually quit, and was replaced, but the tach/dwell meter and vacuum gauge are still the originals! :)
 

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Steve, I read some place that thru frame brake line connectors can have an effect on the brake line pressure. Don't know if you have them or not. It sounds more like what Vall is talking about, but you never know.
 

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One other thing I thought of also Steve! Check to see if the booster/system is holding vacuum. The system should be sealed well enough to hold a vacuum for 3 minutes after the engine is off, or power is off to the pump. If it doesn't stay over 16" vacuum once the pump is off, then there may be a leak somewhere in the system that's causing a vacuum loss and creating the hard pedal issue.
 
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