New Bike! (with some teething issues?)

So after years of slogging along on a mountain-style Dawes bike, my nice cycling-enthusiast brother has fixed-up his old Trek road bike with brand new wheels and cables and things, and I have a new bike!

I'm a bit concerned about a few things and I know he put a lot of effort and labour into fine-tuning the thing, so I don't want to upset him too much with my qualms but:

- chain sometimes slips off down the larger back cog if I go too far into the easy gears. Caught myself in some situations while attempting to go uphill. I tried tightening the L screw at the back a little but it didn't work

- I'm finding it quite painful to ride - lots of pressure on my hands and my upper back/lower neck. Think maybe I need to adjust the seat. My brother is roughly the same height as me, maybe a half inch shorter, so I'm surprised it would need changing. He insists this is something you get used to (I've never ridden a road bike til now). If I use my core, sit back in my seat, it takes the pressure off, but it also means I have trouble reaching the brakes. Maybe I need to experiment with a higher seat?

- Most worryingly are the brakes. If I'm riding with my hands 'low', as in gripping the circular bits of the bars, I can brake quite well, but if I've got my hands in the 'higher' position, as my brother showed me, I don't feel like I can get enough leverage to brake in a hurry. I slow down immediately, but keep going a few seconds before I can stop. Bit worried what would happen if someone were to walk out in front of me.

Concerned, I tried a friend's bike and found the brakes were a lot easier to press down upon. With my bike, I have to press down with all my strength and with all four fingers, and even then it's not a very satisfactory stop. My friend said the mechanism of the break handles might be a bit stiff?

As I say, my bro is very convinced he fine-tuned the bike to perfection. He's really proud of it, and I'm keen to make sure I'm happy with it, obviously. I've been so looking forward to going on longer rides, but it's rather painful to ride and I'm worried about stopping distances. I'm sure a big part of this is getting used to riding a drop-bar, but I've been on three decent-length rides now and I'm not convinced that's the whole problem.

Any tips on the above? I'm useless with bikes

Generally after putting a bunch of new components on a bike; it will need retuning after a bit. Cables stretch, threads settle etc. so I would expect to do a bit of a tune up after very few rides of it bedding in.

As for the comfort, you could try bringing the saddle forward and/or flipping the stem so it points up rather than down. This will raise you body and make it more comfortable by shortening the effective top tube length. Worst case - get a shorter stem or one with more rise. This is generally a core strength thing, so your alternative is to do some yoga or Pilates. It’s not a natural position for he human body so it takes some work to build it up.

Oh, and whilst you may be the same height as your brother you may have subtle differences in the ratio between the lengths of you upper and lower bodies - it’s worth fiddling around with the saddle height.

- chain sometimes slips off down the larger back cog if I go too far into the easy gears. Caught myself in some situations while attempting to go uphill. I tried tightening the L screw at the back a little but it didn't work

iirc you need to turn that L screw *anti-clockwise* instead so that the jockey wheels line up with the largest sprocket

Concerned, I tried a friend's bike and found the brakes were a lot easier to press down upon. With my bike, I have to press down with all my strength and with all four fingers, and even then it's not a very satisfactory stop. My friend said the mechanism of the break handles might be a bit stiff?

have a close look at the brake cables and if they look a bit old, then you probably have a bit of a build-up of muck inside the cable outers which is stopping the cable from pulling freely. if that's the case, try and spray some wd40 or similar into those tubes to flush them out a bit, though you'll probably want to replace them (inners and outers) in the near future

cheers guys.
as far as I know, the brake cables are brand new. most of the bike is new apart from the frame and brake pads (which bro reckons still have a fair bit of life left in them). I don't think it's the brake pads themselves because if I put my hands into the low-position, it brakes fine, but in the top position it's a bit of a struggle.

yeah, worn brake pads wouldn't make the braking feel stiff, just a bit spongy instead. how far do you have to pull the brake levers before they start biting on the rim? if you need to have them full on to get any proper braking effect, then you could try tightening up the brake cable by pulling it through a bit more at the brake end.

I think I have to pull it a fair bit to be honest, but yeah the brake leavers do feel just a little stiffer than my friend's bike where it felt fairly easy

...or try winding the barrel adjuster wotsit out at the lever end obv a bit, depending on the design of the lever xp

ugh phone typing, mangler of language

so I've now:
-raised seat and handlebars. it's slightly comfier I think
-experimented with the L screw but the chain keeps slipping off. I'll take it to a shop and see if they can help
-tightened the brake cables. it's meant I don't need to press down as far but I still don't feel like I can get the bike to stop fact enough - it slows down but still coasts along a little bit before a complete halt. this puts me off going at speed

I'll take it to a bike project and see what they say