GMCforum: GMCnet » Brakes Acting Up

Home » Public Forums » GMCnet » Brakes Acting Up

Show: Today's Messages :: Polls :: Message Navigator

Brakes Acting Up [message #353369]

Tue, 31 March 2020 12:18

uprooted

Messages: 72
Registered: December 2019
Location: Hilliard, OH

Karma: 0

Member

Hey all,

I recently replaced the Master Cylinder, all rubber lines, front calipers, rear wheel cylinders, pads, shoes, two new drums for intermediate axle, new springs inside the drums. The issue: I get GREAT pedal feel when the engine is off. Can practically stand on it and feels very firm. Virtually no downward travel before you hit the 'firm' wall. However when you start the engine and drive, the pedal will travel about 1/3 of the way down before it starts braking (then the coach does stop well once the brakes actuate), then when you release the pedal, it returns upward only until that same 1/3 point, then is VERY slow to come all of the way back up. The other day, the pedal would not come up at ALL. It stopped just fine, but I had to keep my left foot nearby in order to pull the pedal back up to get rolling again. That lasted for that short drive, and now today it is back to 'normal' - 1/3 pedal travel, and slow to return to upward position. What could be causing this?

Corey P / Hilliard, OH / 1974 Glacier 26' / 3.70 FD / ION Wheels

Report message to a moderator

Re: Brakes Acting Up [message #353374 is a reply to message #353369]

Tue, 31 March 2020 13:49

6cuda6

Messages: 975
Registered: June 2019

Karma: -6

Senior Member

Did you bleed the master cylinder before you installed it? Did you bleed it again after you installed it? Did you adjust the rear brakes up manually? All 4 drums?

Rich Mondor, Brockville, ON 77 Hughes 2600

Report message to a moderator

Re: Brakes Acting Up [message #353376 is a reply to message #353374]

Tue, 31 March 2020 14:42

uprooted

Messages: 72
Registered: December 2019
Location: Hilliard, OH

Karma: 0

Member

Yes, yes and yes. MC was blend until no bubbles, then bleed at each wheel probably 4 times. The pedal is very firm when the engine is off.

Corey P / Hilliard, OH / 1974 Glacier 26' / 3.70 FD / ION Wheels

Report message to a moderator

Re: Brakes Acting Up [message #353378 is a reply to message #353369]

Tue, 31 March 2020 16:27

6cuda6

Messages: 975
Registered: June 2019

Karma: -6

Senior Member

Bleed the master again on vehicle....sounds like you might have air in the secondary (rear btakes)....when you have a hard pedal with engine off it doesnt always mean all the air is out....it just means engine off you cant put enough pressure on the pedal to overcome the front brake circuit. IMO

Rich Mondor, Brockville, ON 77 Hughes 2600

Report message to a moderator

Re: Brakes Acting Up [message #353379 is a reply to message #353369]

Tue, 31 March 2020 18:28

Tilerpep

Messages: 404
Registered: June 2013
Location: Raleigh, NC

Karma: 7

Senior Member

Not knowing how you bled the rear...I have only had success with gravity bleeding the rear - slow but consistent. Put an upward curve or loop of clear hose on one rear bleed port at a time and let it sit for a few hours or long enough to significantly drop in the master cylinder. Refill and do the next one.

1975 Glenbrook, 1978 Royale rear bath Raleigh, NC

Report message to a moderator

Re: Brakes Acting Up [message #353380 is a reply to message #353378]

Tue, 31 March 2020 18:31

uprooted

Messages: 72
Registered: December 2019
Location: Hilliard, OH

Karma: 0

Member

How do you recommend? Build pressure, then crack the flare nuts briefly? Disconnect lines from MC completely and run the tubes back to the reservoir? Use the little plastic plugs that came with the MC and let bubbles escape up through reservoir?

Corey P / Hilliard, OH / 1974 Glacier 26' / 3.70 FD / ION Wheels

Report message to a moderator

Re: Brakes Acting Up [message #353381 is a reply to message #353378]

Tue, 31 March 2020 18:39

uprooted

Messages: 72
Registered: December 2019
Location: Hilliard, OH

Karma: 0

Member

I thought I had read on this forum that the combination valve ensures that the rears build pressure first. If that's the case, how would i get firm pedal because i haven't overcome front circuit?

6cuda6 wrote on Tue, 31 March 2020 17:27

Bleed the master again on vehicle....sounds like you might have air in the secondary (rear btakes)....when you have a hard pedal with engine off it doesnt always mean all the air is out....it just means engine off you cant put enough pressure on the pedal to overcome the front brake circuit. IMO

Corey P / Hilliard, OH / 1974 Glacier 26' / 3.70 FD / ION Wheels

Report message to a moderator

Re: Brakes Acting Up [message #353385 is a reply to message #353381]

Tue, 31 March 2020 19:09

6cuda6

Messages: 975
Registered: June 2019

Karma: -6

Senior Member

Usually the first piston chamber in the master is the front brakes. (On some masters you can tell because the biggest fluid chamber is fronts)

Yup bleed on the vehicle....have someone inside....you outside....grab 2 wrenches that fit the flare nuts...crack lines loose and snug up....have assistant start pushing pedal and then you crack lines....have them take their time as you need to close the flares BEFORE they bottom.the pedal and backstroke the pedal....do that until no air comes out....once that done try the pedal engine running....if no better move to the furthest point and start bleeding there.....until no air.

11uprooted wrote on Tue, 31 March 2020 19:39

I thought I had read on this forum that the combination valve ensures that the rears build pressure first. If that's the case, how would i get firm pedal because i haven't overcome front circuit?

6cuda6 wrote on Tue, 31 March 2020 17:27
Bleed the master again on vehicle....sounds like you might have air in the secondary (rear btakes)....when you have a hard pedal with engine off it doesnt always mean all the air is out....it just means engine off you cant put enough pressure on the pedal to overcome the front brake circuit. IMO

Rich Mondor, Brockville, ON 77 Hughes 2600

Report message to a moderator

Re: Brakes Acting Up [message #353394 is a reply to message #353385]

Tue, 31 March 2020 22:11

uprooted

Messages: 72
Registered: December 2019
Location: Hilliard, OH

Karma: 0

Member

My basic understanding of how a master cylinder works is that any pressure that you build on the primary circuit is transmitted through the secondary piston to the secondary circuit. And if there were air in either circuit you would feel it as soft pedal with engine off. Or maybe there is something else unique to the motorhome I am not considering? Or my understanding of how a master cylinder works is not accurate?

Any vehicle I've ever done brake work on, if there is air in either circuit the pedal is soft with engine off. Im not saying there's not air in there but the symptoms don't seem to make sense to me.

Is it possible the booster is failing in a way that is causing there to be vacuum even when the pedal is released? And could this explain why the pedal is slow to return only when the engine is running?

6cuda6 wrote on Tue, 31 March 2020 20:09

Usually the first piston chamber in the master is the front brakes. (On some masters you can tell because the biggest fluid chamber is fronts)

Yup bleed on the vehicle....have someone inside....you outside....grab 2 wrenches that fit the flare nuts...crack lines loose and snug up....have assistant start pushing pedal and then you crack lines....have them take their time as you need to close the flares BEFORE they bottom.the pedal and backstroke the pedal....do that until no air comes out....once that done try the pedal engine running....if no better move to the furthest point and start bleeding there.....until no air.

11uprooted wrote on Tue, 31 March 2020 19:39
I thought I had read on this forum that the combination valve ensures that the rears build pressure first. If that's the case, how would i get firm pedal because i haven't overcome front circuit?

6cuda6 wrote on Tue, 31 March 2020 17:27
Bleed the master again on vehicle....sounds like you might have air in the secondary (rear btakes)....when you have a hard pedal with engine off it doesnt always mean all the air is out....it just means engine off you cant put enough pressure on the pedal to overcome the front brake circuit. IMO

Corey P / Hilliard, OH / 1974 Glacier 26' / 3.70 FD / ION Wheels

Report message to a moderator

Re: Brakes Acting Up [message #353410 is a reply to message #353394]

Wed, 01 April 2020 07:09

6cuda6

Messages: 975
Registered: June 2019

Karma: -6

Senior Member

Without getting into a big debate as to how the master works as their are just to many models etc...lets just treat them as 2 separate circuits connected by a rod between the master cylinder pistons. So if the piston at the end of the rod or furthest away from the pedal has no air in the system but there is air in the first piston circuit your pedal will drop until you build pressure on that piston at the end of the rod. Now if you have air in that end piston circuit but no air in the first piston circuit, the one closest to the pedal, you will have a rock solid pedal because you are working on the closest circuit to the pedal and never get to the second one really.[this is a very general explanation]

Now you also have to remember that you dont have alot of mechanical advantage with this vehicle set up if the booster is not working....so much so that people are adding a vacuum pump, a sensitized booster or even going to a hydro-boost in case of engine off conditions...that tells me that NO boost = HARD pedal....so its not surprising the pedal is hard with the engine off. So this also answers your second question....do you have a bad booster? Nope or you pedal wouldn't be applicable with the engine on.

The only other possibility is a bad master cylinder....which these days is rare.....possible but rare.

So to sum it up as i suspect you had working brakes before this adventure.....if you bench bled the master properly, replaced and adjusted your drums properly and bled all the air out of your system properly you would have a good pedal vehicle running...but you replaced them all and im going to take your word for it that you adjusted the drums properly so that leaves you with air in the system as your first troubleshooting step.

The only other possibility is you have a leak someplace but im sure you checked for that plus your pedal holds, i assume, with pressure engine off.

GO do it!!! Very Happy

uprooted wrote on Tue, 31 March 2020 23:11

My basic understanding of how a master cylinder works is that any pressure that you build on the primary circuit is transmitted through the secondary piston to the secondary circuit. And if there were air in either circuit you would feel it as soft pedal with engine off. Or maybe there is something else unique to the motorhome I am not considering? Or my understanding of how a master cylinder works is not accurate?

Any vehicle I've ever done brake work on, if there is air in either circuit the pedal is soft with engine off. Im not saying there's not air in there but the symptoms don't seem to make sense to me.

Is it possible the booster is failing in a way that is causing there to be vacuum even when the pedal is released? And could this explain why the pedal is slow to return only when the engine is running?

6cuda6 wrote on Tue, 31 March 2020 20:09
Usually the first piston chamber in the master is the front brakes. (On some masters you can tell because the biggest fluid chamber is fronts)

Yup bleed on the vehicle....have someone inside....you outside....grab 2 wrenches that fit the flare nuts...crack lines loose and snug up....have assistant start pushing pedal and then you crack lines....have them take their time as you need to close the flares BEFORE they bottom.the pedal and backstroke the pedal....do that until no air comes out....once that done try the pedal engine running....if no better move to the furthest point and start bleeding there.....until no air.

11uprooted wrote on Tue, 31 March 2020 19:39
I thought I had read on this forum that the combination valve ensures that the rears build pressure first. If that's the case, how would i get firm pedal because i haven't overcome front circuit?

6cuda6 wrote on Tue, 31 March 2020 17:27
Bleed the master again on vehicle....sounds like you might have air in the secondary (rear btakes)....when you have a hard pedal with engine off it doesnt always mean all the air is out....it just means engine off you cant put enough pressure on the pedal to overcome the front brake circuit. IMO

Rich Mondor, Brockville, ON 77 Hughes 2600

Report message to a moderator

Re: Brakes Acting Up [message #353412 is a reply to message #353369]

Wed, 01 April 2020 07:41

Michael Leipold

Messages: 318
Registered: April 2011
Location: Greensboro NC

Karma: 2

Senior Member

I have a very similar situation.
Firm pedal - engine off
Semi-firm with engine running, but the pedal does not return and I can feel the brakes dragging.
The rubber gasket on the booster is dried out, I already replaced the elbow and that improved it some.
If I wiggle the elbow so air does not leak past the gasket, the pedal returns to full height.

I could be wrong, since I have not replaced the rubber gasket, but I think that slight air leak is what is keeping the pedal from returning. Which I will now add to my "Isolation Shopping List"

On a similar note, a friend just had the booster replaced in her truck, they did not touch the brake lines.
Her pedal would not return and the brakes dragged.
The pedal stayed depressed to the point that it activated her brake lights, causing the battery to drain overnight.

My vote: Brake Vacuum Booster

1973 GMC 26' Glacier - Unknown Mileage - Has a new switch pitch transmission with Powerdrive Smile

Report message to a moderator

Re: Brakes Acting Up [message #353414 is a reply to message #353412]

Wed, 01 April 2020 08:15

6cuda6

Messages: 975
Registered: June 2019

Karma: -6

Senior Member

Yup very possible as well....but its easier to tackle a bleed at this point and eliminate tjis simple task off the list...IMO.

Michael Leipold wrote on Wed, 01 April 2020 08:41

I have a very similar situation.
Firm pedal - engine off
Semi-firm with engine running, but the pedal does not return and I can feel the brakes dragging.
The rubber gasket on the booster is dried out, I already replaced the elbow and that improved it some.
If I wiggle the elbow so air does not leak past the gasket, the pedal returns to full height.

I could be wrong, since I have not replaced the rubber gasket, but I think that slight air leak is what is keeping the pedal from returning. Which I will now add to my "Isolation Shopping List"

On a similar note, a friend just had the booster replaced in her truck, they did not touch the brake lines.
Her pedal would not return and the brakes dragged.
The pedal stayed depressed to the point that it activated her brake lights, causing the battery to drain overnight.

My vote: Brake Vacuum Booster

Rich Mondor, Brockville, ON 77 Hughes 2600

Report message to a moderator