RV Brake issue

[USNRET]

A co-worker (long time aircraft mechanic) has a fairly recently purchased RV of some sort, a Chrysler product I think, that has spongy brakes. The previous owner had the master cylinder replaced but the problem remains. What say you?
First brake application is spongy but the second feels good. As long as you hold pressure after first application they work fine.
Brakes have been bled to death.

Good vacuum at booster.

Engine off has hard pedal as it should be.

Could it be bad check valve in vacuum line?
Could it be that whoever replaced the master cylinder did not set proper booster to master cylinder rod end gap?

Ideas?

 

 

[JJkizak]

11 hours ago, USNRET said:

A co-worker (long time aircraft mechanic) has a fairly recently purchased RV of some sort, a Chrysler product I think, that has spongy brakes. The previous owner had the master cylinder replaced but the problem remains. What say you?
First brake application is spongy but the second feels good. As long as you hold pressure after first application they work fine.
Brakes have been bled to death.

Good vacuum at booster.

Engine off has hard pedal as it should be.

Could it be bad check valve in vacuum line?
Could it be that whoever replaced the master cylinder did not set proper booster to master cylinder rod end gap?

Ideas?

 

 

 

Assuming they are drum brakes:

1...The drums have been turned too much for the diameter of the new shoes.

2...The pistons are too old.

3...The drums have been poorly turned.

4...The wrong compounds on the shoe liners, some of the cheap stuff works awful.

5...The shoes not properly adjusted

I had this problem with old corvettes and ended up replacing the drums, shoes and pistons.

JJK

 

[jason str]

Its air in the system. None of the above mentioned will cause a spongy petal.

 

Try a power bleeder if you have the option.

 

If the master cylinder was not properly bench bled before installation it may be the problem as well.

[CECAA850]

34 minutes ago, jason str said:

If the master cylinder was not properly bench bled before installation it may be the problem as well.

Very few people actually take the time to do this and it's critical.

[JJkizak]

Let us know what had to be done to fix it.

JJK

[ricktate]

I heard of this type of problem before on my JH car club site. Has been traced to rubber inside brake lines going to the wheels has deteriorated. So it allows the fluid to stretch the line on first push.  We also had problems of dragging brakes do to this also, the rubber would expand then shrink down and not let the fluid go back. The above ideas are good ones also just have to figure it out.  

[CECAA850]

1 hour ago, ricktate said:

Has been traced to rubber inside brake lines going to the wheels has deteriorated.

Many times that's from people doing brake jobs and letting the calipers dangle off of the brake hoses instead of tieing them up when replacing pads or turning rotors.

[USNRET]

Thanks for the input guys. Bill will be back this Friday and I'll share the info with him and see if he made progress.
 

[joessportster]

Now this is Ironic, I had a couple at my house all morning looking at my 1991 Hawkins Diesel Pusher, on the way home from test drive I started smelling the wonderful acrid hot brake shoe smell, get out and the front hubs are HOT (Not smoking but not far from it I am sure)  It has sat for months and I am sure I have corrosion not allowing the air brakes to release fully

 

Help coming by in the morning to remove the Semi size wheels, pull hubs and have a look

 

To your issue I have had this on cars in the past and it was either  air trapped in master cylinder (Those damb things can take forever to bleed properly) Or the line had air trapped. I would start at the Master and bleed the hell outta it, then move to the wheels.