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Old 05-26-2022, 08:55 PM   #1
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76 Dodge M500 Motorhome Brake Issues

Hello:

I'm getting rid of my 76 King's Highway RV which has been sitting at least 8 years. The guy taking it got it started and it move front and back but has no brakes. He tried bleeding the brakes, but no fluid comes out on all 4 bleeder screws, and yes master cylinder has fluid. So, he figured it was the master cylinder and pick one up at NAPA. After installed the new master cylinder, same thing, no fluid comes out on all 4 bleeder screws.

The guys on a buget and instead of just throwing this and that on this RV I thought I asked here for anyone that had an experience like this or someone with brake knowledge that can put us in the right direction. In other words, your advice is very much appreciated.

Thank you in advance!!
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Old 05-27-2022, 05:35 PM   #2
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Have you checked to make sure the bleeder screw holes are open. They do tend to plug. Small drill bit run through them and some air will usually open them back up.
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Old 05-27-2022, 06:41 PM   #3
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Under brake pedal pressure there are several hundred psi of fluid pressure. Make sure the bleeder is open enough.

Is the brake pedal going to the floor? Did you bench bleed the new master cylinder?
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Old 05-28-2022, 01:25 AM   #4
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Quote:
Originally Posted by ihc1470 View Post
Have you checked to make sure the bleeder screw holes are open. They do tend to plug. Small drill bit run through them and some air will usually open them back up.
Yes, the bleeders screws are open.
Thanks
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Old 05-28-2022, 01:29 AM   #5
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Quote:
Originally Posted by 38Chevy454 View Post
Under brake pedal pressure there are several hundred psi of fluid pressure. Make sure the bleeder is open enough.

Is the brake pedal going to the floor? Did you bench bleed the new master cylinder?
Yes, bleeders are open.
Yes, pedal goes to the floor.
Yes, bench bleed the master cylinder before installed.
Thanks!
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Old 05-28-2022, 05:15 AM   #6
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Looking at M500 chassis online, one site shows a hydroboost unit in it.

Do you have 2 rear axles, if yes, then one axle is controlled by the hydroboost. When they fail, they can suck the brake fluid into the engine and burn it.
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Old 05-28-2022, 05:24 AM   #7
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Assuming no leaks along the length of the brake lines themselves, remove the lines from the master cylinder and one at a time remove the other end of each lie from the calipers or wheel cylinders and blast air through them. Its not unheard of for the metal lines to rust up and clog. If air blasts through the lines, then the next step is to remove the bleed screws completely from the calipers and cylinders and blast air through them to see if it comes out the line connection. Be sure to leave the caliper and pads mounted and only remove the bleed screw and brake line or else you'll pop the piston out and then you're rebuilding it. Same thing applies for the rear wheel cylinders.
Frankly if this new owner is preparing to drive anywhere more than a few slow miles, they would be better served to just replace all the hard and rubber lines and be done with it. If there is so much blockage within the brake system that fluid won't pass, there is a leak & complete failure coming very soon especially when the extreme heat and pressure builds up from a couple of stops. Sorry this person is short on cash but going cheap and lazy with brakes on a large heavy vehicle like an RV is not a good idea.
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Old 05-28-2022, 07:49 AM   #8
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When the master is bench bled then it should be ready to pump fluid. If you start pumping it while on the vehicle the brake fluid level should go down in the reservoir and start filling the lines. If that isn't happening, then I would crack a brake line at the master to verify that it is trying to push fluid. It should be able to push fluid somewhere. If there is a proportioning valve in the system it should at least push fluid to the front or rear.
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Old 05-28-2022, 10:42 AM   #9
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Quote:
Originally Posted by twinboat View Post
Looking at M500 chassis online, one site shows a hydroboost unit in it.

Do you have 2 rear axles, if yes, then one axle is controlled by the hydroboost. When they fail, they can suck the brake fluid into the engine and burn it.
No, 1 rear axle. Thanks!

Quote:
Originally Posted by Tidy Tabby View Post
Assuming no leaks along the length of the brake lines themselves, remove the lines from the master cylinder and one at a time remove the other end of each lie from the calipers or wheel cylinders and blast air through them. Its not unheard of for the metal lines to rust up and clog. If air blasts through the lines, then the next step is to remove the bleed screws completely from the calipers and cylinders and blast air through them to see if it comes out the line connection. Be sure to leave the caliper and pads mounted and only remove the bleed screw and brake line or else you'll pop the piston out and then you're rebuilding it. Same thing applies for the rear wheel cylinders.
Frankly if this new owner is preparing to drive anywhere more than a few slow miles, they would be better served to just replace all the hard and rubber lines and be done with it. If there is so much blockage within the brake system that fluid won't pass, there is a leak & complete failure coming very soon especially when the extreme heat and pressure builds up from a couple of stops. Sorry this person is short on cash but going cheap and lazy with brakes on a large heavy vehicle like an RV is not a good idea.
Fluid is coming out the master cylinder lines. Going to tell him to blast air through the lines. I do understand brakes are important in a RV, but not to sure the new owners does, but he's not leaving here until it is fix. Thanks!

Quote:
Originally Posted by cadillac_al View Post
When the master is bench bled then it should be ready to pump fluid. If you start pumping it while on the vehicle the brake fluid level should go down in the reservoir and start filling the lines. If that isn't happening, then I would crack a brake line at the master to verify that it is trying to push fluid. It should be able to push fluid somewhere. If there is a proportioning valve in the system it should at least push fluid to the front or rear.
So, if the proportioning valve is not operating it should still allow fluid to the bleeders? Thanks!
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Old 05-28-2022, 12:45 PM   #10
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Does it have a big brake booster behind the master cylinder ?

If not, it may still have a hydro boost down the chassis rail.
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Old 05-28-2022, 05:10 PM   #11
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You have gotten some good advice. I will add that I have seen bleeder screws with clogged ports on vehicles that have sat for extended periods. The cure is to remove one at at time, hold your finger over the hole, and have someone pump the brakes. If you get fluid, drill out the port, *gently* just to clear the existing holes. I use a hand chuck, but you can use s power drill.

Rinse and repeat for the other three corners. You can get new bleeder screws, of course, but drilling them out is a lot faster and slightly less expensive.
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Old 05-28-2022, 05:17 PM   #12
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1976 vintage ID be replacing/rebuilding wheel cylinders and/or calipers. The pistons could be frozen
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Old 05-29-2022, 04:35 PM   #13
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Finally got the brakes working. Originally, he said the bleeder screws looks good. Today he brought someone that knows what he was doing, and he clean/open all breeders screws and the brakes bleeded fine and got brakes.

Thank you all for your help!!!
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