brake question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

nvrdone · 03-22-2017, 10:54 PM

my '49 3100 has the stock drum brakes at all 4 wheels. I decided to up grade & change the stock master cylinder to a mustang m/c with power booster. mostly to improve the stopping power and not worry about a complete brake failure if a line broke or wheel cylinder failed.
Big mistake. now the truck stops worse than it did before the change. I replaced all the lines with the same size as stock with 2 separate circuts. the kit came with an extension that needed to be welded to the end of the brake pedal short end to improve the ratio. I also added a 10# residual check valve into the front line.
Now the pedal goes within about 2" of the floor before there is any reaction and its scary to try and stop quick.
All the drums have been turned, new shoes installed and wheel cylinders replaced trying to solve the problem.
do I need to find a m/c with the same bore as stock ? any one have any ideas ? im about ready to install a stock m/c with out the booster.
thanks

yossarian19 · 03-23-2017, 01:13 PM

Sounds likely that you have air in the lines. It can be a pain to bleed them with the frame mounted master cylinder. That's why I went to a firewall mounted kit.

**Dan in Pasadena** · 03-23-2017, 02:14 PM

Over the years I've read that lots of guys have problems bleeding brake lines with the frame mounted MC's.

Without a Miti-Vac you could try jacking up the rearend as high as you possibly can to encourage air to rise to the "top" making it (theoretically) easier to get it to bleed out. Then reverse it and jack the front end up to the hilt to do the front. Just a suggestion.

nvrdone · 03-25-2017, 12:24 PM

--------------------------------------------------------------------------------

everyone: thanks for the input. after reading all the info, seems like ive done everything recommended. so it looks like the next step will be to change the m/c to a drum / drum one. any ideas on a part # ?

yossarian19 · 04-03-2017, 03:18 PM

A drum / drum MC won't fix this.
A low pedal is one of a handful of things
not enough fluid volume being moved for the brakes (this is not your problem, as disc brake MC's move more volume than drums do)
push rod / linkage has too much slack before contacting the MC piston
air in the lines
hydraulic leak, internal or external
volumetric expansion of a brake line that's about to explode
somebody let me know if I'm missing anything
You have something else going on than needing a different MC.

03-22-2017, 10:54 PM	#1
nvrdone Registered User Join Date: Dec 2011 Location: Moxee WA Posts: 1,425	brake question my '49 3100 has the stock drum brakes at all 4 wheels. I decided to up grade & change the stock master cylinder to a mustang m/c with power booster. mostly to improve the stopping power and not worry about a complete brake failure if a line broke or wheel cylinder failed. Big mistake. now the truck stops worse than it did before the change. I replaced all the lines with the same size as stock with 2 separate circuts. the kit came with an extension that needed to be welded to the end of the brake pedal short end to improve the ratio. I also added a 10# residual check valve into the front line. Now the pedal goes within about 2" of the floor before there is any reaction and its scary to try and stop quick. All the drums have been turned, new shoes installed and wheel cylinders replaced trying to solve the problem. do I need to find a m/c with the same bore as stock ? any one have any ideas ? im about ready to install a stock m/c with out the booster. thanks

03-23-2017, 01:13 PM	#2
yossarian19 Registered User Join Date: Sep 2011 Location: Nevada City, CA Posts: 908	Re: brake question Sounds likely that you have air in the lines. It can be a pain to bleed them with the frame mounted master cylinder. That's why I went to a firewall mounted kit. __________________ "Over my head" 1957 Chevy 3200, big rear window & 6 lug. Front disc, power steering, Vortec 4.8 / 4L60E swap, hydro boost brakes & patina.

03-23-2017, 02:14 PM	#3
Dan in Pasadena Senior Member Join Date: Nov 2008 Location: Glendora, CA Posts: 6,331	Re: brake question Over the years I've read that lots of guys have problems bleeding brake lines with the frame mounted MC's. Without a Miti-Vac you could try jacking up the rearend as high as you possibly can to encourage air to rise to the "top" making it (theoretically) easier to get it to bleed out. Then reverse it and jack the front end up to the hilt to do the front. Just a suggestion. __________________ '55 Big Window Shortbed, Drive-It-&-Work-On-It slid down the "slippery slope" to a Frame-Off Rodstoration! LQ4/4l85e/C4 IFS/Mustang 8.8 rearend w/3.73's Dan's '55 Big Window "Build" - Well, Kinda!

03-25-2017, 12:24 PM	#4
nvrdone Registered User Join Date: Dec 2011 Location: Moxee WA Posts: 1,425	Re: brake question -------------------------------------------------------------------------------- everyone: thanks for the input. after reading all the info, seems like ive done everything recommended. so it looks like the next step will be to change the m/c to a drum / drum one. any ideas on a part # ?

04-03-2017, 03:18 PM	#5
yossarian19 Registered User Join Date: Sep 2011 Location: Nevada City, CA Posts: 908	Re: brake question A drum / drum MC won't fix this. A low pedal is one of a handful of things not enough fluid volume being moved for the brakes (this is not your problem, as disc brake MC's move more volume than drums do) push rod / linkage has too much slack before contacting the MC piston air in the lines hydraulic leak, internal or external volumetric expansion of a brake line that's about to explode somebody let me know if I'm missing anything You have something else going on than needing a different MC. __________________ "Over my head" 1957 Chevy 3200, big rear window & 6 lug. Front disc, power steering, Vortec 4.8 / 4L60E swap, hydro boost brakes & patina.



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