Technical Master Cylinder size and Braking

As I continue to learn, I thought the easier option was going to be adding a booster to make braking a little easier. However, I have my MC and brake pedal mounted underneath of the frame. Disc brakes all around. I've been researching and wondering what options I have or maybe I am overlooking something. Stopping is not so much of an issue as my car stops well, but sitting at a light and in drive, you really have to lay into the brake pedal to make sure you don't move at all. The few times I was on gravel it was very difficult to stop.

Space is certainly an issue with adding a booster, and a 7inch would be the max but even that would be tough. Not only the diameter but also moving my MC back alot further making it very difficult to access for my floor panel. I have also seen mix reviews on the 7inch actually doing much. Then I started reading about bore size.

So my question is, is there something I am overlooking or should check to make braking easier? I am fine with sticking with my set up just really trying to understand more of what maybe I need.

 

Auto? Sounds more like a torque converter problem. Do you know your stall?

 

Why is it more difficult to stop on gravel? Is the car pushing the front wheels? If that's the case your front/rear brake bias is weighted too heavily to the front, and also as @Tman said it sounds like you might not have enough stall in your converter.

 

Yes. TH350 trans. mild build 355 small block. I do not know the stall, trans came with the car, I put the new motor in. Could you elaborate on what I need here?

@Ebbsspeed Yes in gravel in gear its more of sliding a bit trying to stop. If I throw it in neutral, whether on street or gravel, stops fine. So originally the car had a 305 in it but was shot, but never had any issues with braking. Once I put the mild 355 in, thats when braking became more difficult. Like I said, I can slow down and come to a stop but you really have to have your foot into the pedal to keep the car for inching forward. But in neutral, no issue.

 

There are a million threads on this topic so I just picked one. https://www.jalopyjournal.com/forum/threads/no-stall-torque-converter.1157095/

Here is what I Googled, it makes your search easier "torque converter stall speed/HAMB"
I think I had a 2500 or 2800 stall when I built my old T touring.

 

The post you picked has almost the exact problem. Here I am and it might not even be anything to do with braking. Appreciate the info...I wasn't even thinking about the converter. Ill start reading up for sure. Is there a quick way to tell how much stall my converter? Would any markings be on it?

 

I would take a look but you probably have to talk to a ****** guy

 

Agree totally on the converter issue; got the t shirt on the one! But check your idle rpms before getting too deep into it as too high on this can have a similar effect.

Chris

Sent from my SM-T515 using The H.A.M.B. mobile app

 

My idle is always around the 800-1200. So I think I am good there right? Im sure I am going to go down a worm hole looking at the forums for info, but what stall should I be looking at? I know I have 168 tooth flexplate, but just a quick search as I never purchased a TC, I didn't realize how many options are out there.

 

That is an awfuly fast idle speed. Can't you get it down to 900 or so? 800 rpm would even be better. That would also solve your problem. With that fast an idle I'm sure you have problems stopping or standing still with the transmission in gear.

 

Idle should be under 1000. Sounds like your motor is racing at idle. No wonder it is hard to hold it with the brake.

 

Maybe you have a vacuum leak or any other lean condition if you can not get your idle low enough. I´ll second that 2k is pretty high, even a race only engine should idle at 12-1500rpm, a mild 355 should easily idle at 650 -700 rpm once warmed up I suppose.

 

Apologies guys, I was typing from my phone....my idle is around 800-1200......not 2k.

 

Sounds pretty high nevertheless, but I guess that depends on the engine. I have a 350hp 350 that idles about i think 700 in park, so a bit higher in neutral. Warmed up obviously.

Chris

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***ume you mean 700 in Drive?

 

Absolutely! Stupid me!!

Chris


Sent from my SM-T515 using The H.A.M.B. mobile app

 

I am probably 1000-1200 cold...700-900 warmed up. I truly do not think my idle rpms are the issue. But I am still in the building stage so dont have many miles on the new engine. Now I am concerned about the TC as it sounds like from a few forums ive read, this is more of my issue than the braking.

If anyone else has some recommendations on TC's I should use to possible solve my problem let me know a part number. I will continue to look this this forum as there is always a ton of great info.

 

Can you contact the person you bought the engine from? You said a warmed up 355, but having an idea of the engine specs would give you an idea of what stall speed you’d need. Some folks consider warmed up as an intake and headers, others 300 HP, others 400HP, etc. all (IMO) would use a different stall

 

Just curious. Is this the same ****** that was in the car behind the 305?

 

As far as what converter you need many factors come into play. As stated, what is in the engine, weight of the vehicle, gearing etc. How warmed over of a sbc are you talking here?

 

Yes same ******. 9 inch rear and I believe the gearing is 3.5. I have 33x21.5-15 mickey Thompson tires on the rear. The car is light...Id say 2000 lbs max give or take. I will try and reach out to the guy I bought the motor from, but it is a very mild build, don't have the specs (I only paid 800 for the motor and redid the top end gaskets). Then put new covers, pullies, carb, etc on. Engine never goes over 180 degrees warmed up and like i said it will idle around 1000. I definitely need to get a timing light on it, which is on the list.

Since I put the new motor in I've driven less than 500 miles. I wanted to figure this out before spring time and honestly never gave the TC a thought to be the problem. But the more I read, sounds like a higher stall is what I need. Also, the more i read I realize a lot goes into the selection so sorry I do not have definite answers. The guy I basically bought this from bought it from a guy before he p***ed away and didn't have much info so I am learning just about every time I go to mod something.

 

I'd think about the torque converter as issue #2.
You need to know the all in timing (around 34 degrees at 2500 rpm with vacuum advance disconnected and plugged). Reconnect vacuum advance.
Adjust idle mixture and idle speed (hopefully about 700 rpm in drive).

If for some reason that doesn't resolve the issue of brakes holding the car in drive, THEN, consider other issues.

 

Disc brakes are not self energizing like drum brakes and require more pressure BUT the fact that everything worked fine with the old 305 engine tells you that the problem is not necessarily the brakes. The stall test we used at the dealership was to have a fully warmed up engine and transmission, Hook up a tach if you don't have one on the dash, chock the wheels and stand on the brake pedal as hard as you can. Open the throttle slowly and whatever rpm the tires break loose or the engine won't gain any more RPM that is the stall speed for that converter / engine combination. The same converter can have a different stall speed with different engines based on the torque produced and at what RPM the torque peaks. A diesel ***mins will require a lower stall converter that a V 10 gas engine converter. Even though both engines make lots of torque and use the same basic transmission the peak torque on the ***mins is 1000 rpm lower.

 

I had a problem with a car pushing when stopped at a light, with the brakes on and car in gear, when on snow. It ended up being the proportioning/combination valve had failed. This was a stone stock 6cyl/auto daily driver. The rear brakes were not working yet it seemed to stop fine in the dry.

If the car stopped well with the 305 yet won't stop well with the 350, then either the brakes got messed up during the swap (pinch a line to the rear maybe?) or the cam is larger/not as mild an engine as you have been told and the idle speed is too high for your stock converter.

Did YOU actually drive it with the 305 or are you being told by the previous owner that the brakes worked fine when they had it?

 

Correct, before I change anything I will get the timing right. Again, before all this info I was thinking it was soley an issue with maybe needing a booster. Before I rip anything apart I will make sure my timing is where it needs to be.

@borntoloze Yes I drove the 305 a good bit before the swap. I do not have a combination valve on my MC. Like i said, both engines slow down/come to a stop just fine, its more about the car 'pushing' as you described at a light.

 

Another question on the combination valve-Would this be a good idea to add being the huge difference in tire sizes I have? Would that help at all with the creeping I have?

 

I have never run one on all drums. My T had 5s and 10s, my 54 had the same. The 54 Sedan was half a ton heavier and ran a stock converter. The T had the high stall. We did install those on the disk drum cars that came out of the shop.

 

If the car stopped just fine without a combination valve before the engine swap, then I would say you don't need one. I believe your car is either idling too high for the converter (too big a cam or not properly tuned, vac leak, timing etc), or you've pinched a brake line that feeds the rear brakes when you jacked the car up and put it on stands to do the engine swap, preventing the brakes from working to their full potential.

If it worked fine before, you need to find out what changed that caused the problem. Adding a proportioning valve may help (by allowing you to direct more pressure to the rear brakes) but it won't solve the true cause of your issue.

My previous example of my car pushing on snow (caused by the combination valve failure in my case) was just to show that you may have a rear brake problem, regardless of whether you have a combination valve or not.

It is unlikely the problem is with the brakes but stranger things have happened ... master cylinders don't let you choose when they fail

I bought an 81 Regal and the brakes didn't feel right ... turns out the master wasn't actuating the rear brakes. Changed the master and solved the problem.

Regarding your tire differences. I owned a tubbed 70 Nova with skinnies on the front and 31" Mickey's on the rear (disc/drum with factory combination valve and factory master) and never had an issue with brakes.

 

Thanks for all the input as always, this site has helped me more times than I could count over the last 3 years. Ill start with getting a proper tune and see where I am at.

 

Before you spend any money.............
Go somewhere and drive at 30mph and stop normally, then repeat it with the trans in neutral. If it stops well/much better when in neutral then the brakes are fine.
Before looking at torque converters, try to find out why your idle speed is so high. Do you have a cam that won't idle if you lower the rpm, or is your carb just not adjusted properly? Get a vacuum gage and see what it tells you about the engines vacuum. Mark your engines settings so you can return to the original adjustment. Double check your timing to see if advancing/retarding helps or hurts any. Write down what you started with and make a mark on the distributor base and engine so you can return to that if need be. Lastly buy a book from Amazon on the particular carb you have and it should tell you how to adjust the carb properly. Until you get the best idle speed, I would not think about changing your converter. Torque converters have specific "stall speeds" which is the rpm where they begin to engage the engine to the transmission. If you increase the stall speed then the engine can idle at a higher speed. The downside is that when driving, your engine will need to be at the higher stall speed when driving on the xway. So if you have a car that turns 1800 rpms at 70 mph ......and you change to a higher stall speed, you may have to run the engine faster when on the xway. It depends on how you have your car set up as far as gearing and tire size. So its not as simple as just putting a higher stall converter in so it can idle.