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Old 04-23-2014, 03:13 PM   #1
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Brake booster testing?

I have a 69 Glastron on a Dodge m-300 Chassis and the two Midland 490g brake boosters appear to not be working. Does anyone have any experience with these units and can advise on how to test for proper function ? I did find that even tho my rv is titled a 69, it has a 68 chassis and my book does not cover this unit very well. Appreciate any help available.
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Old 04-24-2014, 09:41 AM   #2
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Not much help with your problem but I can tell you on my 64 caddie the break booster had a vacuum leak. I ended up replacing it and now have power breaks again. We could hear a faint hissing around the pedal area.
Best of luck to ya, Tim
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Old 04-24-2014, 09:56 AM   #3
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Before you go tearing the brake boosters apart check to see if there's vacuum at the inlet port. It should be a right angle plastic one way valve with about a 1/2" diameter hose coming from the engine. You can pop the hose off the fitting and put a vacuum gauge on the hose. Also check to see that the valve operates properly. It's supposed to open when vacuum is applied from the hose, but not let it bleed off when the engine is off.

On a vehicle of that age it's not uncommon for a hose to leak. It doesn't necessarily have to be the hose going to the vacuum booster. Any substantial vacuum leak will limit the amount of vacuum going to the booster. If you find you have poor vacuum check all the hoses coming from the intake manifold. If you can't find a leak block off the hoses one at a time. It may be the component they're hooked to is leaking not the hose itself. If you still have poor vacuum disconnect all the hoses and plug the ports. Then put the gauge directly on a port to the intake manifold. If you still have poor vacuum it could be either a bad intake manifold gasket or a bad valve.
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Old 04-25-2014, 08:06 AM   #4
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Brake booster.

Thanks for the replies. I did check out the check valve and it was clear and seems to be working ok. I got antsy and pulled the front booster and dis assembled. The bellows area was full of brake fluid! apparently the seals on the bellows shaft failed. I have found a place to get kits for them and am in the process of awaiting parts. I think I am gonna get brakes real soon.
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Old 04-26-2014, 07:47 PM   #5
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Most of this information is from the 69-73 Dodge Motorhome Service manual however I assembled it into one place Dave's Place - M300 (Early) 1969-71 for a given configuration (Dave's Place - Dodge MH Brake Info)

At top left are links to booster, master cylinder, Front and Rear brake data.

Dave
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Old 04-27-2014, 05:34 PM   #6
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Midland boosters.

Dave: I found the site several days ago and appreciate all the great info assembled there. I am trying to digest some tech details in regard to the direction of assembly of the Slave cylinder cup direction of installation. I think I have it figured out. Time will tell and again thanks for the great info.
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Old 04-27-2014, 09:43 PM   #7
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Flared section of cup goes in the bore first. When pressure is applied, the fluid pushes the flare outward to maintain a seal. Flat side is against the piston.

Dave
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Old 04-28-2014, 08:02 AM   #8
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Thanks Dave: That is the way I had it figured. Jim
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Old 04-28-2014, 08:07 AM   #9
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There are two kinds of brake booster, both can be tested while parked,

Vacuum boost: press and release the brake pedal several times the brakes should be very hard.. Now while holding the pedal start the engine, Pedal shold move toward floor and feel more normal.

HydroBoost.

Hydro boost has two ways it can be powered, EITHER an electric pump or by the power steering pump.

While parked, Press pedal, You should HEAR the pump motor start and the pedal will first stop or seem to then continue SLOWLY by comparison to go down part way to the floor.

With the engine running it should go down much faster.

And that's all there is to testing them... From the driver's seat.

NOW: a proper technician with the needed tools may be able to do a more "In depth" test.. but that is the basic test.
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