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Topic Boards => Dodge - Chrysler Chassis => Topic started by: cbowles on August 07, 2022, 09:33 AM

Title: Hard brake pedal but not immediately
Post by: cbowles on August 07, 2022, 09:33 AM
Having rebuilt the brake system on a 1975 Landau, Dodge M500 a couple years ago I thought the next one would be easier but its got me baffled.
I am fixing another old motorhome because I just can't help myself. This one is a Travco 270 on a 1974 Dodge M500 chassis (motorhome year is 1975). It has the exact same system as the Landau, dual Hydrovacs, front disc, rear drum. The symptom is this:
I have perfectly normal brake operation for the first 1 minute or so of braking. Then the pedal gets hard and brake action diminishes greatly, just like a poor vacuum condition or non functioning booster.

Here is what I have done:

Replaced master cylinder with the a new NAPA part number 4959

Replaced both front calipers and soft lines and installed new pads.The rotors are not grooved, look good.

Replaced both Hydrovac boosters with overhauled items (Cardone) from NAPA, part number 51-8055. Identical to the ones I purchased from Alretta for the Landau. One of the boosters I had purchased from Alretta developed a punctured diaphragm just after the 1 year warranty expired so I wanted to try a different supplier hoping for better longevity.

Replaced vac hard and soft lines, with proper vac line not fuel hose

The brake light comes on as it should, during start only.

All 4 wheels stop when up in the air and brake is apllied

The rear brakes look good with almost new shoes and the wheel cylinders free.

I installed an electric vacuum pump and an T inline, plugged the manifold vac connection for troubleshooting purposes. Check valve is good. The electric pump makes 20 inches of vacuum and it stays between 18 and 20 for a long time after pump shuts off so no apparent leaks. When I apply the brake while parked with the vac pump on I can see the vac drop is minimal at first, it takes many pumps to show the vacuum going down. While driving it is the same, good vacuum around 18 to 20 inches regardless of how much or how long brake is applied.

When I apply the brake pedal while parked and the vac pump off I get the usual hard pedal, short travel and as I switch on the vac pump the pedal drops drown a little(not much). A wooshing sound and a clicking sound can be heard (the pistons in the hydrovac?) when applying brakes

I studied Dave's brake information again (thanks Dave, tremendous help) but can't figure out what is wrong here. New master, new calipers, new boosters, good vac, no vac leaks, no air in system, normal pedal travel. What could be wrong with this?
Title: Re: Hard brake pedal but not immediately
Post by: cbowles on August 15, 2022, 08:48 AM
Well I didn't get any replies, seems like everybody else is also stumped with this.
 I'll pull out the boosters again and see what I can see, since the hydraulic side of the brake system works fine it must be in the booster somewhere. Perhaps the regulator in one of the new ones was defective.
Title: Re: Hard brake pedal but not immediately
Post by: Mlw on August 15, 2022, 12:47 PM
I think you need to call Geoff at Alretta, you mentioned that allready.

The man is a wizard with breaks and it's better to talk to somebody who knows what he talks about.

Title: Re: Hard brake pedal but not immediately
Post by: yellowrecve on August 15, 2022, 09:05 PM
I went through 4 rebuilt hydro vacs in a short time. One that I received came out of the box cracked. I decided to convert to 1989 Ford E350 power brakes. To was relatively easy. Mounted to the same bracket that held the dodge master cyl. I already had the ford.  I fabricated as needed. That was in 2013 and still going.
Title: Re: Hard brake pedal but not immediately
Post by: mytoolman on August 18, 2022, 12:50 PM
I am experiencing brake issues also not like yours where the pedal is hard....I did all new rear except drums their hubs and the un ubtainium self adjuster parts. Fortunately those parts were all fine. I REPLACED both hard lines that tie the dual rear wheel cylinders in each rear corner. Replacing them was a challenge.

 The  compound bends in that steel brake tube that have the steel tubing  line only clearing the leaf springs by about a 1/4 inch at one of those bends took me eight weeks of down time to learn what to do. The original steel line there was formed in a jig during the manufacturing process. I couldnt find anyone that had such a set up to form the bends required. The steel tube would kink before I could make the sharp bends that where necessary. Finally I learned a technique that solved the issue.

 Melt "lead ingot"
   pour the melted ingot into the steel brake line tube. Once the lead hardens inside the brake line tube you will form to fit the steel line to nicely fit the groove in the rear brake backing plate and also be able to make those short 90 degree bends where the steel line screws into the wheel cylinder. Heat up the steel line to make the lead liquid again and remove the molten lead from the line.

 50 year old steel brake tube lines will have rust inside of them. I could not remove one of mine that is what caused me to have to replace the one and while I was at it I did the other. The one that came off un broken served as my template

The other area not mentioned by you is the special high pressure  rubber hose that the brake boosters use....Maybe the inner wall of said hose has developed a break and now has a internal flap that keeps the pressure from releasing etc. Or maybe its collapsing etc.   
Title: Re: Hard brake pedal but not immediately
Post by: cbowles on September 19, 2022, 06:57 AM
I pulled the boosters and both of them had brake fluid leaking into the can with the diaphragm, there should not be brake fluid on that side. These were "remanufactured" by Cardone in November of 2021 so they didn't even hold up for 1 year. The originals lasted 40 years

The 2 boosters I purchased from the auto parts store are covered under a lifetime warranty so at least I get another set but its still a lot of work. I am working on installing them and will report back. If this doesn't fix it I'll replace the master cylinder, again.
Title: Re: Hard brake pedal but not immediately
Post by: Oz on September 19, 2022, 07:36 AM
Didn't last a year?  I've read a few people refer Cardone as a rebuild source but haven't heard any other complaints.
Title: Re: Hard brake pedal but not immediately
Post by: eXodus on September 21, 2022, 05:03 AM
my rebuilt Cardone Hydroboost Brake Booster exploded 10 minutes after I completed a 4 hour repair.
A rebuilt Powersteering pump - was never sealed right. Dripped fluid right out of the box.

I'm staying away from rebuilt parts now, quality control the last few years is not great.  Spend lot of money and time on those and failed.

The good thing about our old RVs - they are built on truck chassis - and the Classic Truck community found many good alternatives which fit.

Look what the truck people figuring out, usually there is a new Part from a recent Model Year which fits your application - is new OEM and much improved in performance.
There is no need to match what's currently in there - you get something better.
Title: Re: Hard brake pedal but not immediately
Post by: cbowles on September 21, 2022, 04:44 PM
Could you be more specific, with part numbers perhaps?
I am ready to throw in the towel with the "remanufactured" parts.
Again, I seem to have a bad hydrovac booster as I just found out by isolating each booster. When I plugged the vac line to the front booster the rear brakes alone did a great job of slowing down the motorhome. When I plugged the vac line to the rear booster I have very little to no braking from the front discs. So it must be the front booster I recently replaced. What junk and it wasn't exactly cheap.

Only on other option possible- the metering valve is holding back way too much pressure to the front brakes....seems unlikely since I can see it move out when applying the brakes
Title: Re: Hard brake pedal but not immediately
Post by: eXodus on September 22, 2022, 05:11 AM
I'm not Dodge chassis guy, so I don't know the exact part numbers
Just wanted to share my experience from the Chevy chassis side:

The big OEMs reuse parts all over their product lines, after getting frustrated finding parts for a RV-Chassis - I started looking for parts for similar years of Pickup and Medium duty trucks of the same manufacturer. Since there had been built many more trucks then RVs - there is larger need for aftermarket and more people are trying out different solutions. Folks put new upgraded parts on their Show-Room Ready Classic Trucks all the time.

I just googled real quick and saw the the M500 chassis has similar parts as the D350 Dually Pickup.

So then I google "D350 brake booster upgrade"

and found this:

Title: Re: Hard brake pedal but not immediately
Post by: cbowles on September 30, 2022, 03:59 PM
Problem solved. It was the master cylinder and it was covered under warranty by NAPA as this was a new manufacture part, not a "rebuild"- it has a lifetime warranty. The master cylinder is made in Mexico, part number M4959 and is the correct part for the 1974/75 Dodge M 400/500, it crosses over to the original Dodge Part number. Don't know why this one gave up so quickly, hopefully the replacement will hold for the next 40 years
Title: Re: Hard brake pedal but not immediately
Post by: mytoolman on October 01, 2022, 10:57 AM
awesome that the issue is solved. It really is difficult today to get systems back up and running. We think we have done a repair and can move on to the next part of a particular component in a system only to have to be baffled when the system doesnt work correctly when we have virtually replaced every component in said system. Its very challenging to go back and diagnose everything to find the new part we just installed and believed to be not an issue turns out to be defective and the cause of the tail chasing we were forced to endure....
Good job persevering through to solve the challenge. 
Title: Re: Hard brake pedal but not immediately
Post by: eXodus on October 01, 2022, 04:43 PM
Basic Rules:

Never assume a new part is good.
Only change one part at a time.