TheSamba.com :: View topic - Spongy Pedal After Converting to Dual Circuit MC

Sun Sep 21, 2014 6:26 pm

Here's what I have:

AC Industries wide 5 front disc brake kit and early Bus rear brakes (reduction box transaxle).

Everything worked fine when I used the single circuit master cylinder that came with the disc brake kit. It's an early Bus master cylinder.

I decided to switch to a dual circuit master cylinder for safety. I used a late model Bug master cylinder.

I've pressure bled the brakes several times and continue to get a soft, spongy pedal. The pedal pressure is too soft to be safe. It goes to the floor if I really press hard.

I'm wondering if the dual circuit master cylinder is too small of a bore to handle the disc brakes and the Bus rear wheel cylinders. I believe it's 19mm compared to 22mm for the single circuit master cylinder I removed. :(

Any ideas?

Sun Sep 21, 2014 8:16 pm

Had the same problem.. Same set up that you are running except... I used the larger MC and still had a spongy pedal after the change over to the AC Disc...

I searched about every web site I could find. Then I also called AC Industries and talked to the Tech dept. (Great help)

What I did to solve my problem..

1- Jacked up the rear of my Baja so that my rear wheel cylinders were above the MC.

2- Installed a 2lb. residual valve in the rear brake line in front the "T"...
**** Yes I'm running the big drum brakes on the rear, and "everyone" says use a 10lb valve for drum brakes... But... AC said to try a 2lb. valve and after bleeding all the wheels a couple times with this set up... I couldn't be happier with my brakes now....

3- That's my story and I'm sticking to it.......

Sun Sep 21, 2014 8:41 pm

I'm running drums all around but I just replaced the single circuit back into a dual master made by AC Industries as well. I replaced the two rear wheel cylinders with AC ones, which I noticed were larger than the ones I took out. I've bled it clear at all wheels and my pedal is like you described. Sort of firm with a gentle press but if you push harder it gives and pumping it actually makes the pressure decline.

Sun Sep 21, 2014 9:01 pm

I had a squishy pedal for 7 years that stopped about 1/2" from the floor. I could never figure out why i had squishy pedal syndrome. People said "did you bleed the master cylinder before you put it in?"
Well i ignored that comment bc i figured i had bleed the whole system so many times that the air HAD to of gotten out by now.
Last week i put my master cylinder back on (*after extensive front end work) and I decided to bench bleed it before putting it back in the car.
World of difference. I have never had such good hydraulic pressure in the 7 years i have had a vw.

So i have to ask, Did you bench bleed the master cylinder?

I have stock drums on front, Empi disks on the rear with a 10# residual valve before the T

Sun Sep 21, 2014 9:04 pm

thesatelliteguy wrote: I had a squishy pedal for 7 years that stopped about 1/2" from the floor. I could never figure out why i had squishy pedal syndrome. People said "did you bleed the master cylinder before you put it in?"
Well i ignored that comment bc i figured i had bleed the whole system so many times that the air HAD to of gotten out by now.
Last week i put my master cylinder back on (*after extensive front end work) and I decided to bench bleed it before putting it back in the car.
World of difference. I have never had such good hydraulic pressure in the 7 years i have had a vw.

So i have to ask, Did you bench bleed the master cylinder?

I have stock drums on front, Empi disks on the rear with a 10# residual valve before the T

I didn't bench bleed the mc, but I did pressure bleed the system when I installed it. This method has always worked well for me in the past. It worked with the previous single circuit MC.

Sun Sep 21, 2014 9:07 pm

dowha wrote: Had the same problem.. Same set up that you are running except... I used the larger MC and still had a spongy pedal after the change over to the AC Disc...

I searched about every web site I could find. Then I also called AC Industries and talked to the Tech dept. (Great help)

What I did to solve my problem..

1- Jacked up the rear of my Baja so that my rear wheel cylinders were above the MC.

2- Installed a 2lb. residual valve in the rear brake line in front the "T"...
**** Yes I'm running the big drum brakes on the rear, and "everyone" says use a 10lb valve for drum brakes... But... AC said to try a 2lb. valve and after bleeding all the wheels a couple times with this set up... I couldn't be happier with my brakes now....

3- That's my story and I'm sticking to it.......

That sounds like good advice.

What size master cylinder do you use? I know EMPI makes a 20.6mm dual circuit master cylinder for disc brake applications.

What larger rear brakes do you use? Type 3?

Sun Sep 21, 2014 9:44 pm

Yep... Type 3 rears..

I ordered the MC from AC with the Volvo type reservoir... I think it is 20.6mm...

Sort of did a bench bleed too... With MC mounted... pumped & held the pedal, while a buddy opened & closed the brake lines @ the MC..

Mon Sep 22, 2014 9:36 pm

dowha wrote: Yep... Type 3 rears..

I ordered the MC from AC with the Volvo type reservoir... I think it is 20.6mm...

Sort of did a bench bleed too... With MC mounted... pumped & held the pedal, while a buddy opened & closed the brake lines @ the MC..

Great. I'll give this a try. Your Type 3 wheel cylinders are about the same size as my Bus ones.

Where did you get your residual valve? Is it a metric size?

Mon Sep 22, 2014 10:21 pm

Appletree Auto here in Michigan The one I used was std.1/8 NPT...

Any buggy supply place carries them..

Tue Sep 23, 2014 10:05 pm

dowha wrote: Appletree Auto here in Michigan The one I used was std.1/8 NPT...

Any buggy supply place carries them..

Cool. It looks like EMPI makes one for metric fitings. I might give that a try this weekend if one of the local shops stocks it.

Sat Oct 04, 2014 5:24 pm

So, I went ahead and installed an EMPI 2 pound residual pressure valve in the line to the rear brakes.

The pedal still feels a little spongy, although not as bad as before.

While taking it for a brief test drive, it seemed like the car nose-dived really bad every time I stepped on the brakes. It felt like only the front brakes were working.

I'm not sure if I have the pressure valve installed correctly. It had an arrow stamped into it that I assumed pointed in the direction of flow...away from the master cylinder.

Being an EMPI part, of course it did not include instructions.

This is getting frustrating.

Sun Oct 05, 2014 8:22 am

Could the 19mm master cylinder be too small?

Sun Oct 05, 2014 8:35 am

fladge- I also had same problems after converting my car to dual mc and AC-dics brakes- tried alot of things and scrathed my head a lot!!! i finely said screw it i'm taking it out and driving it.... i drove around in desert for awhile-up hills, down hills, bounching it thru the whoops, then parked on a hill with front pointed down hill and bled rear brake for the million time and had air come out! i drove some more and repeated and after a few times got a nice firm pedal..... just my experince....

Sun Oct 05, 2014 9:26 pm

race-desert wrote: fladge- I also had same problems after converting my car to dual mc and AC-dics brakes- tried alot of things and scrathed my head a lot!!! i finely said screw it i'm taking it out and driving it.... i drove around in desert for awhile-up hills, down hills, bounching it thru the whoops, then parked on a hill with front pointed down hill and bled rear brake for the million time and had air come out! i drove some more and repeated and after a few times got a nice firm pedal..... just my experince....

Maybe I need to give that a try.

I bled the brakes again with the rear jacked up and seem to have a slightly firmer pedal now. Still not as firm as I think it should be.

Sun Oct 12, 2014 1:34 pm

Update:

After driving the car around for a while and checking the adjustment again, followed by another bleeding of the brakes, everything seems to be working well.

The pedal seems much firmer and the braking seems better. Less of a nose dive situation like before.

The pedal still has a little give to it, but it feels better than the rock-hard pedal feel of the 22mm Bus master cylinder that came with the kit.

For review, my brake system is made up of the follwing parts:

AC Industries wide 5 front disc brake kit (Uses Rabbit calipers)
Early 1960's Bus rear brakes with Bus 22mm wheel cylinders.
Late Bug dual circuit 19mm master cylinder
Empi 2 pound residual pressure valve for the rear brakes.